To calibrate: Yudkowsky didn't sign because he feels a temporary moratorium doesn't go far enough, and he published a piece in Time [0] with a stance that a shooting war to enforce a ban on AI training would be justified, to prevent the extinction of the human race.
I'm not convinced that he's wrong.
[0] https://time.com/6266923/ai-eliezer-yudkowsky-open-letter-no...