Earlier quoted context omitted.
What would they do? I'm not sure there's an actual threat to follow through on. They can't block a forked compiler from, for example, using official packages without close-sourcing their own compiler (to prevent it from bringing in whatever change makes it work again). At that point, why bother respecting the threat? A forked compiler isn't going to get maintainers? A package depending on the forked compiler isn't go…
It's still worth knowing that they're being jerks about it, since that falls outside of the social (but obviously not the legal) aspects of open source.
Not really, IMO. Forking a project used to be a really aggressive thing to do back before GitHub made it so common among younger waves of developers.
If you go to Evan and say, "Look, I want to use your thing but do it my way." and he says "No, I have a clear vision of what the thing is and it's not that. KTHXBYE." I don't think that's "being a jerk".