Definitely not true about models knowing jj. I've had latest opus and gpt fail at revsets and fileset syntax, even hallucinating subcommands like jj move (maybe it existed before, interface is not stable). Luckily it's easy enough to not need them most of the time.
Yes, I use Claude with jj often, and it occasionally tries to use older commands like move. Most of the time it gets it right for me though, often plugins and such say to use git explicitly, and that's where it tends to stray.