I read it as a joke; chalk is necessary but not sufficient for the language to gain GATs.
What other major things are needed?
In my understanding, Chalk's immediate goal is to be able to reproduce the existing trait system. Achieving that goal does not mean that GATs just automatically happen; they'll need to be implemented in Chalk, and then all the other polish work that happens with new major features needs to happen. There's a ton of work other than "chalk exists."