Would it be a valid for someone to take "GPL v2" (with no or later version) clause, and re-license a derivative work as "GPL v2 or later", and then another entity takes that derivative work and re-licenses it as the "GPL v3"?
That seems to be what's happened here. The b2 software is a strict "GPL v2". There is no later clause. Then WordPress has re-licensed their derivative work as "GPL v2 or later". Now we are talking about a GPL v3 derivative work.
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.
That seems to be what's happened here. The b2 software is a strict "GPL v2". There is no later clause. Then WordPress has re-licensed their derivative work as "GPL v2 or later". Now we are talking about a GPL v3 derivative work.