canada
You stipulate that "the intended meaning is not in doubt" and that they "mak[e] it clear" they "mean the opposite." What matters is the meaning that the ordinary person would infer from the words. That meaning is what will be assessed for defamation.
The impugned statement may convey more than its literal meaning
Courts recognize that meaning is contextual. It is not enough for a defendant to argue that a defamatory statement is literally true, if the defamatory sting of the comment stems from non-literal meaning.
In Canada, (see Weaver v. Corcoran, 2017 BCCA 160, at para. 71):
Words may convey a defamatory meaning literally, inferentially or by legal innuendo. Literal meaning is conveyed directly; inferential meaning, indirectly; and legal innuendo, by extension based on extrinsic facts. ...
Where the literal meaning of words is in issue, it is unnecessary to go beyond the words themselves to prove that they are defamatory. Where a claim is based on the inferential meaning of words, the question is one of impression: what would the ordinary person infer from the words in the context in which they were used? Both literal and inferential defamatory meaning reside within the words, as part of their natural and ordinary meaning. In contrast, where legal innuendo is pleaded the impugned words take on defamatory meaning from outside circumstances beyond general knowledge, but known to the recipient.
Terminology
As an aside, to address an issue arising in the comments to the question: whether a statement is defamatory is separate from whether it is true or false. In Canada, for example, a statement is defamatory as long as it "would tend to lower the plaintiff’s reputation in the eyes of a reasonable person" (Grant v. Torstar Corp., 2009 SCC 61). And in the United States, courts also separate the two concepts. See for example, Pegasus v. Reno Newspapers, Inc., 118 Nev. 706 (Nev. 2003)
The general elements of a defamation claim require a plaintiff to prove: "(1) a false and defamatory statement...
Many defamatory statements are not grounds for tort liability in defamation because they are true.