1. Matt just claimed it wasn't about software at all. So we've now moved on to saying "well it's about code, but not GPL code".
Progress, i guess?
2. The agreement does not limit it to non-GPL code in any way, so i'm really not sure where you are getting that from. The plain terms cover all of automattic (et al) software, which includes the stuff that has GPL code in it.
The normal way such an agreement would be written to cover only non-GPL pieces would be to say it does not attempt to modify or change rights you get elsewhere, blah blah blah.
It includes no such clause.
If you've got meaningful parol evidence that it was meant to cover only non-GPL code, great, let's see it.
(since the agreement is both badly written and not fully integrated, it would be valid to do so).
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.
And
"WP Engine will cease and desist from forking or modifying [non GPL code]"
You are misreading it.