How can a plugin be GPLv3 when the core is GPLv2?

//edit: apparently the core is v2 or later.

Correct, it's gplv2 or later. v2/v3 GPL compatibility is also sort of an abstract thing - it gets talked about a lot (IE do the terms have to actually conflict as applied to a given situation or only conflict in theory) , but you also can't file a lawsuit with a claim of "GPL incompatibility". You'd have to file a claim for breach of contract or infringement (or both).

This matters because if, for example, your main incompatibility is around the patent pieces (that's one of them), and there are no patents anywhere involved, you may have a hard time convincing a judge that there is really a breach of contract.

I do not believe it is legally "GPL v2 or later" at all. The original b2 license was GPL v2. There was no or later version option in the original b2 license. Given that WordPress is a derivative work, it has to keep the same or compatible license. Which "GPL v2 or later" is not.

Note how the original license is GPL v2 at [0], then the "or later" header is added much later at [1] seemingly out of nowhere.

[0] - https://github.com/WordPress/WordPress/blame/04c9051a7d765cb...

[1] - https://github.com/WordPress/WordPress/commit/8cbd92f9f8269a...