My understanding of Matt's position, and what that term sheet he posted is talking about, is that he feels that WP Engine needs a trademark license to cover what it's currently doing. He feels that their usage of the marks exceeds what's generally allowed under the "everyone can use it this way" terms that WordPress.org offers. His position isn't
just that they can't use the trademark to apply to their forked version, it's that they can't use it as they currently are even if it wasn't forked.
So Matt's now offering these terms: a license to use the trademarks as they currently are, in exchange for concessions. The concessions include that they'd agree to not use their GPL-granted right to fork the code. This sounds like an agreement that would hold up, assuming he's right about the trademark policy.
Again, I have no idea about the actual legalities of trademark usage. It's entirely possible that Matt is wrong about them not being allowed to do what they're currently doing with the trademarks, but it sounds like lawyers will be arguing about that.