I'd trust Matthias to know when GTK is ready, and it's a signal to the community that it's definitely time for testing and porting. But he can still make micro bumps. :-)
Indeed: as it is said in the post, they had to give up on a feature because they could not implement it in time for their .99 version and they locked themselves in that system.
That's less about the version number, more about the time-based release train. If you're not ready to go, you can't get on.
GTK/GNOME started doing time-based releases every six months 18 years ago. Hasn't missed a beat.