Now that's an ahistorical conspiracy theory. Those diverse desktop environments contributed hugely to GTK, GNOME just didn't use their work or consider it helpful unless it directly related to their desktop... and of course none of that work will relate to their desktop. Nobody is going to fully "kiss the ring" unless they get something out of it, and even back in the GTK3 days it was plainly clear that GNOME didn't care about you if you didn't care about GNOME.
Now, GNOME's "coup" or "killing" of GTK is completely fine by Open Source standards. Even encouraged. I don't stand against the concepts of what they're doing, but they could have done a lot better than fighting third-parties tooth-and-nail. GNOME should be a proud project that leads the GNU movement, and instead it was reduced to a bunch of squabbling supremacists that made their userbase an adversary. I say all that as someone who quite likes modern GTK and writes apps in it.