That's not because of the GPL. The GPL has little to do with barring access to a platform on which code is published. Arguably, if a copy existed elsewhere, say GitHub, then WPEngine is free to use that code according to the GPL.
In other words: once code is published with the GPL and someone has a copy, the original creators can do little to nothing to stop them from using said code however they see fit. That's what drives forking.
In the same vain, original creators always have, and will have, the freedom as rights holders over creative works, to change the license on new versions published. Of course, the caveat being holding the rights over contributions made by third parties (hence the existence of contested contributor agreements).
The real issue here is a for-profit entity driving the governance of a non-profit entity. There's not just the ethical but also legality at play here. And this has little to do with copyright.
In other words: once code is published with the GPL and someone has a copy, the original creators can do little to nothing to stop them from using said code however they see fit. That's what drives forking.
In the same vain, original creators always have, and will have, the freedom as rights holders over creative works, to change the license on new versions published. Of course, the caveat being holding the rights over contributions made by third parties (hence the existence of contested contributor agreements).
The real issue here is a for-profit entity driving the governance of a non-profit entity. There's not just the ethical but also legality at play here. And this has little to do with copyright.