I don't think we're going to agree on this one.
We agreed that a contract was offered which, if accepted, would revoke the GPL right to fork.
We agreed that the GPL doesn't allow such a revocation of the GPL right to fork (even if it is part of a contract).
We agreed that even attempting such a violation (e.g. by offering such a violating contract) is itself a violation of the GPL.
Honestly, it sounds like we're in violent agreement. The only specific disagreement you've expressed is in the best way to word something among several options which you say are equivalent.
Separately, the GPL calls out that you're allowed to place a restriction on your distributed code when it comes to trademarks, in that you're not required to license your trademark. I.e. you're allowed to distribute code that's GPL-licensed but that it's impossible for people to legally use as-provided without either forking it or obtaining an extra license from you. v3 is a lot more explicit about this, but it seems implicit in v2 as well.
You could certainly take the position that any sort of trademark license containing usage restrictions at all qualifies as a restriction under the GPL, so your only choices are to completely allow or forbid usage of your trademarks without any conditions. I disagree with that view.
You've interchangeably described WPEngine's consideration under the contract to be "something", "stuff", and "behavior" – this vagueness seems to be dodging the issue: what specifically, as you understand it, would WPEngine have to give up under the terms of the contract Matt presented, in exchange for usage of the trademarks?