Caveats
The question triggers a lot of subsidiary issues in the course of discussing the core title issue, "Is a copyright license a real right (right in rem)?"
This answer addresses primarily the title issue, because addressing some of the subsidiary issues related to particular kinds of licenses in the second paragraph of the question (e.g. irrevocable licenses and GPL licenses) would just be too much to squeeze into an already somewhat lengthy answer.
The answer also seems to be seeking an answer from a civil law legal system perspective (addressed briefly and inconclusively in a final section of this answer), rather than from a common law perspective, but this answer does, at least, provide a little bit of civil law and comparative law insight, although not as definitively. As explained in Law.SE help:
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united-states
Overview
Generally speaking, in most circumstances, a license is an "in personam" right arising from a contract, and not a property right.
Nota bene, however, that a license is not necessarily a true contract, in the strict sense of legal doctrine. It can also be something less than a true contract, such as a unilateral grant of permission to use a work that is not supported by consideration and does not have the express agreement of two or more parties.
But, the idea which seems to be the premise of the question, that something is strictly a contract right enforceable only in "in personam" proceedings in all circumstances, or is strictly only a property right enforceable only in "in rem" proceedings in all circumstances, is not an accurate statement of U.S. law.
The law generally distinguishes between "in personam" and "in rem" legal proceedings or causes of action, rather than "in personam" and "in rem" rights. It isn't at all unusual, across the board in U.S. law, for the same right to be potentially subject to both "in personam" proceedings which seek relief personally from the defendant, and "in rem" proceedings which seek remedies solely from the property that is the subject matter of the action. The remedy sought in a proceeding normally determines whether it is an in personam or in rem proceeding, not the nature of the right itself (although the nature of the right can influence the kind of relief most commonly sought).
The distinction between "in personam" proceedings and "in rem" proceedings, while parallel to the distinction between contract rights and property rights, also isn't precisely the same thing. Furthermore, sometimes something that is a contract right for some purposes is a property right for other purposes. Many contracts are contract rights between the parties to the contract, but are property rights vis-a-vis third parties.
Ultimately, how a copyright licenses is characterized comes down to what makes sense under the circumstances, in light of the procedural posture in which the question presents itself.
Example
Circumstances where copyright licenses are treated as contract rights and enforced in in personam proceedings
For example, if I were trying to ascertain the statute of limitations for a breach of a copyright license, I would assume that the statute of limitations for a breach of contract would apply, rather than the statute of limitations for conversion or trespass of property or adverse possession, that normally apply to property rights.
Similarly, in most circumstances, I would assume that you could not secure service of process in a lawsuit for breach of a copyright license by publication, something that is only allowed for in rem lawsuits.
And, generally, in a lawsuit for breach of a copyright license, the plaintiff would be seeking money damages and/or injunctive relief, not possession of the license (which only barely makes sense in the case of intangible property although the square peg can be made to fit in that round hole).
Basically, in the context of a lawsuit between a copyright owner and a copyright license holder, a copyright license is usually viewed as a contract right that is enforced in an in personam proceeding.
Circumstances where copyright licenses are treated as property rights and enforced in in rem proceedings
On the other hand, one could bring a lawsuit to quiet title to the underlying copyright, just as you would bring a lawsuit to quiet title to real estate, in cases where there is a dispute over who owns the copyright and subject to what encumbrances. And, in that context, you could name a purported license holder in an "in rem" action to quiet title to the copyright, could serve process in the quiet title action by publication, and this could result in the purported license holder losing their license if it wasn't defended in a timely manner.
Similarly, just as a bank account is a contractual matter between the bank and the customer, but can be considered a property right for other purposes (e.g. in a divorce or a probate case), whether a dispute related to a copyright license is treated as a contractual matter or a property right depends upon the context in which it arises.
For example, suppose that someone dies owning an assignable copyright license that has not expired. This copyright license would be treated as property in the estate of the decedent and could be transferred to a new owner in connection with a probate case, which is (for the most part) an "in rem" proceeding.
Similarly, it would be possible to have an action to determine who owns a license, and while it would be more customary to call that a "declaratory judgment action" rather than a "quiet title action", in this context, they are really identical.
Irrevocable licenses
it seems to me that in the real world, an "irrevocable license"
granted to somebody is truly irrevocable, and thus is binding to the
current copyright owner as well as any future assignee, just like a
real right.
The word "irrevocable license" while commonly used in copyright licensing is something of an oxymoron.
Real estate law compared
In the context of real estate, a "license" is a revocable right to be present on particular real estate which does not create a property right, even if unjust termination of a license to be on property can give right to a breach of contract action against the person granting the license.
For example, a movie theater ticket is a license. It can be revoked unilaterally with or without good cause by the movie theater management as an agent or tenant of the property owner. If you are kicked out of a movie theater by management, you can't refuse to leave because you bought a ticket and avoid a criminal trespassing offense. But, you could sue to get the money you spent on the ticket back if you were kicked out of the movie theater unjustly.
When a right to be present on real estate is irrevocable without good cause, even if it is only irrevocable during a term of years, we call that a lease, rather than a license. A lease is both a contract between the landlord and the tenant, and a property right that is a defense to a criminal trespassing offense if not revoked for good cause using the statutorily required procedures. In theory, you could have a lease that is good for the life of a tenant, rather than a term of years, although this isn't customary in arms length commercial dealings (with a legal or equitable life estate being much more common when this is the intent).
Even so, in real estate, whether a lease is a contract or a property right isn't always so clear. In a more pedantic strict sense, a lease is an estate in real property called a "term of years" followed by a remainder interest. But, it is common place for people to think of a lease as an irrevocable contractually granted right to possession that does not confer ownership or an estate in land (something often reflected in title registration or recording systems, and in real property taxation).
In the law of real estate, there are also some other concepts that are also similar to some irrevocable licenses of a copyrighted work.
An easement or "servitude" grants permission to use real estate that you don't own in specified ways, by specified people, for specified purposes. And, lots of irrevocable copyright licenses are really more analogous to easements or servitudes than they are to true licenses to use real estate. Sometimes servitudes (the more general term that encompasses several different kinds of rights analogous to easements in addition to easements themselves) are considered an estate in real property that is owned, but it is more common to call them "encumbrances" (a category that also includes mortgages and liens). Servitudes are still considered property rights, however, for bankruptcy law purposes, and can't just be discharged as debts without providing adequate security. Servitudes have the same priority as secured debt in bankruptcy and probate cases (which is the highest possible priority in a particular item of property).
In the law of real estate, when you grant express permission to members of the general public to use real estate that belongs to you, without charge, this is called a "public dedication" of that real estate for some kind of use, and a lot of standard form copyright licenses are more analogous to public dedications than they are to licenses.
Now, in the context of copyright law, a license has a somewhat broader meaning, of any express grant of permission to use a copyrighted work which may or may not be subject to conditions and restrictions.
The implications of an assignment of copyright ownership
The linkage the quoted language makes between the fact that a copyright license is irrevocable and its survival of a transfer from one copyright owner to another, isn't really true in either the case of a copyright license, or any of the various licenses, leases, servitudes, or dedications that can be made of real property.
A copyright license, even if it is revocable, continues to bind an assignee of the copyright from the original copyright owner, until it is properly revoked pursuant to its terms. And, generally speaking, both the original copyright owner and the assignee will have liability for breaching the terms of the copyright license if the assignment was made without the consent of the licensee.
The same is true with real estate.
The exception to this would be a transfer of real estate, or of a copyright, respectively, which involuntarily terminates the rights of everyone in the chain of title of the person granting the right to use the property (such as a copyright license).
For example, if real estate is sold for unpaid property taxes, everyone with an interest in the property thorough the owner's chain of title has their rights in the property automatically terminated.
Likewise, if a lawsuit established that the owner of the copyright to a certain software program belonged to Fred Simms who wrote it, and not to SoftCo, which thought it owned the copyright to it as work for hire (that wasn't properly documented) and licensed it, the licenses from SoftCo which never actually owned the copyright would automatically be terminated as a matter of law.
Licenses in bankruptcy and the limits of logic in U.S. law
When someone who has a license files for bankruptcy, the license is part of the property of the bankrupt debtor unless it imposes ongoing obligations on the part of the bankrupt debtor, in which case the license is an executory contract of the person who has the license.
But, I don't know how copyright licenses are treated in bankruptcy when the copyright owner files for bankruptcy.
It would be plausible to call them "executory contracts" which can be either affirmed and adhered to by the bankrupt, or rejected and give rise to an unsecured claim for money damages as an unsecured creditor in bankruptcy.
But it would also be plausible to call them property rights, particularly if they are irrevocable, which cannot be discharged or reorganized away without providing adequate security to the license holders.
As I said, I don't know the details of that U.S. case law.
But an important observation is that knowing how particular kinds of software licenses are classified as property v. contracts for civil procedure purposes, or statute of limitations purposes, or accounting purposes, does not tell you how they will be classified for some other purpose, such as bankruptcy law. You cannot discern the state of U.S. law classifying a right in a particular context from how U.S. law classifies that same right in a different context.
The famous statement about American jurisprudence from Oliver Wendell Holmes, Jr., from his book The Common Law (1881), which is still sound advice for someone trying to understand American law is:
The life of the law has not been logic; it has been experience.
It is often particularly hard for people with STEM backgrounds such as computer programmers, engineers, and scientists, who build their lives around reaching conclusions from long chains of logical reasoning, to come to terms with the fact that you can't rely upon long chains of logical reasoning to get accurate answers to legal questions. American law is extremely context specific and fact contingent, and only rarely faithfully follows general rules on a consistent basis in all contexts.
comparative-law
Non-U.S. jurisdictions
Other common law jurisdictions
The U.S. law analysis above would tend to be true more generally in other common law countries as well. This context specific fluidity of "in personam" v. "in rem" classifications is something that would be quite general in all genuinely common law jurisdictions. Your mileage may vary in a hybrid common law-civil law-other law jurisdictions on a jurisdiction by jurisdiction basis.
Civil law jurisdictions
Civil law countries (e.g. France, Germany, Spain, Japan, South Korea, Mexico), tend to be more rigid in their categorizations and more consistent across different contexts in this kind of matter. Often terms are defined and categories are established in the general part of a civil or commercial code, and then applied rigidly, consistent with the general part, in all parts of the code (and sometimes, in all parts of multiple legal codes).
But, I'm not familiar enough with civil law jurisprudence in this area to provide a reliable answer there as applied to copyright licenses.
Other legal systems
Copyright and licensing in the Western sense aren't natively part of Islamic law, or of most forms of tribal or customary law, and to the extent these concepts exit in countries that use these kinds of law, copyright issues generally arise under Western style statutes or treaties distinct from the overall context of their legal system, much like, for example, the traffic laws applicable to motor vehicles, that didn't exist in the formative periods for these bodies of law.
I also wouldn't venture any firm statements on how this issue would be treated by the law of the People's Republic of China (PRC), which, while it has modernized and adapted to global commercial norms notwithstanding its Maoist Communist roots, and while it still has civil code type statutes on the books, still has contract and property law concepts that are rather more fluid than those of common law and civil law legal systems. In the PRC, neither your contract rights nor your property rights are as absolute as they are in the West and can give way more easily than in the West to considerations of reasonableness, state needs, etiquette, the public good, and the relative positions of the parties in the transaction or legal matter. One generally can't take seemingly absolute pronouncements in PRC statutes at face value, as they are subject to a variety of considerations not present to the same extent in Western legal systems that become evidence when they are applied.
