Why does Howell cite this particular case, Gunn v. Minton, for this
very general principle that everyone knows?
It is the strongest case supporting the proposition since it comes from the U.S. Supreme Court and is one of the most recent U.S. Supreme Court cases to say so.
It is also worth noting that the starting point for a judge's legal research is the set of cases that are cited by the lawyers for the parties in their legal briefs (and the cases cited in any amicus briefs, in cases where those are filed).
Most of the time, the judges just stick to those cases (especially when both sides are citing the same cases for some general legal proposition). But it is very common (i.e. it happens maybe 40-50% of the time), especially for elite judges, such as in a prestigious federal court like the U.S. District Court for the District of Columbia, that a judge will cite a case that stands for the same proposition as the cases cited by the lawyers but is more recent or more squarely on point. If the better case cited by the judge is a standard for the same legal point as the cases made by the parties, this is usually done without further input from the parties.
If the case cited by the judge and found from the judge's own legal research, that is not cited in any of the legal briefs in the case, supports an argument not made by the lawyers for either side, then often (but not always, or even a majority of the time) the judge will ask for the parties to provide supplemental briefing on the relevance of that case before making a final ruling.
Mediocre judges (and judges who are receiving briefing from excellent lawyers) only rarely cite cases not cited by the parties, and when they do, it is usually because they are using boilerplate forms language from their own prior opinions to address the same issues (e.g. the standard of review for a motion to dismiss), or because the lawyers all overlooked a very famous and clearly applicable case.
Isn't there some more authoritative statement, perhaps ultimately
statutory, from which which this principle derives its force?
Not really. It is an inference from the U.S. Constitution and the overall structure of title 28 of the United States Code, but those sources don't say that in so many words.
A citation of a case interpreting a statute or the U.S. Constitution in some sort of context is almost always more authoritative than the bare language of a statute or the U.S. Constitution not embedded in a factual context that clarifies any potential ambiguities in the meaning of the language.
If you cite the bare language of a statute or constitutional provision, it would be possible to argue that one of the words there is being used in a sense of that word that has a different meaning than the one that you are arguing applies (something analogous to the kind of argument that the phrase that "the offender must be stoned" in the Bible means that the offender must be "under the influence of marijuana" rather than that the offender must be "punished by throwing rocks at them"). Citing a case that applies those words in even a minimal context rules out those kinds of arguments against how the statutory or constitutional text should be interpreted 95% of the time.
Or if not more authoritative, then something older? The principle long
predates both Gunn and Kokkonen.
Because a more recent citation reduces the likelihood that the principle has been overruled or modified or distinguished in more recent cases, and reduces the amount of legal research that is necessary to confirm that point (i.e. one need only look at U.S. Supreme Court cases since 2013). The strongest possible authority is usually the most recent U.S. Supreme Court case on point (especially on issues of constitutional law which can only be superseded by a U.S. Constitutional amendment).
Since Gunn is itself directly quoting Kokkonen, why did Howell cite
Gunn and not Kokkonen?
First of all, to be accurate in the attribution of a quotation, and secondly, to show that the principle has been unchanged for a longer period of time (30 years instead of 12 years). See also this Law.SE answer.
Why cite anything at all?
Because legal writing, and court opinions, are supposed to provide authoritative support for every legal proposition made, to the fullest extent possible. Factual assertions are likewise generally supported with citations to the court on appeal or the evidence in the record in a trial court.
Ideally, a court opinion has foundations of law and fact that are all supported by citations and contributes its analysis only to the smallest degree possible for the most obvious conclusions that can be reached from the facts and the law so that it is harder to question. And, the more squarely on point the cases cited are, the easier it is for the opinion to make the case that its ruling is the only possible correct ruling.
To overturn the ruling, someone would either have to conclude that one of the legal cases cited was incorrect, or that one of the factual citations is wrong, or that the application of the law and the facts to the conclusions reached is wrong. If the legal and factual citations are sound then you want to minimize that part of the opinion that is your own analysis, especially to the extent that it is not glaringly obvious.
If a proposition is exceedingly widely adopted and not really subject to meaningful question in the case, however, it wouldn't be uncommon to just cite to the leading case, like Lujan, or to the most convenient reasonably recent case, without bothering to carefully determine what the absolutely most recent controlling case might be.
Jurisprudence about the requirements to survive a 12(b)(1) motion
wasn't changed by Arpaio or by Lujan, was it? If not, why are these
relevant?
Lujan is the U.S. Supreme Court case (which is the highest and best possible authority) that established the current standard of review. A Federal Rule of Civil Procedure 12(b)(1) motion is a motion challenging subject-matter jurisdiction. It is taught in law school and is a precedent familiar to any lawyer who litigates these kinds of cases.
Since the court is citing Lujan, we know that the aspect of jurisdiction being challenged is subject-matter jurisdiction arising from a dispute over standing to sue in a public law matter (i.e. a matter involving governmental action), and we know that the case is squarely on point to this case which is also a dispute over standing to sue in a case arising from governmental action.
Lujan is the case in which the U.S. Supreme Court defined the current standard for when someone has standing to bring a lawsuit challenging the legality of governmental actions, and is the leading case regarding that issue from which all other standing to sue over governmental action cases flow.
Arpaio is a case illustrating that this standard of review continues to be good law in the U.S. Court of Appeals circuit in which the court is located at a much later date to show that the principle has been unchanged for a longer period of time in the relevant U.S. Court of Appeals Circuit without an implicit repeal or overruling or distinction (reducing the time period in which any further legal research could contradict that conclusion). It basically confirms that a panel of the D.C. Circuit Court of Appeals has more recently confirmed that Lujan was still good law as of the time that Arpaio was decided.
There is probably a D.C. Circuit case more recent than Arpaio that reaches the same conclusion, and probably even a more recent U.S. Supreme Court case that reaches the same conclusion, but because the proposition is so widely accepted, the court didn't bother to find those cases.
Arpaio is also more squarely on point than Lujan, which is more general. Lujan articulated the standard to sue the federal government in general, but Arpaio involved standing to sue over a decision of a sitting U.S. President in particular.
Arpaio also established that the U.S. District Court for the District of Columbia (as opposed to some other U.S. District Court) has subject matter jurisdiction over lawsuits concerning the validity of actions of sitting U.S. Presidents (because the White House is located in the District of Columbia), while Lujan did not, which is a side issue of subject-matter jurisdiction that was not really in dispute but could have been raised.
Finally, the citation of a case against President Obama may have been chosen from multiple possible cases, to give the rhetorical effect of applying a legal standard that has been applied to Democratic and Republican Presidents alike.
