I think the other question is how likely he would be to walk away. I think at $0, Musk would likely stick it out at Tesla, since so much of his net worth, ego, identity, and brand is locked up in there. In other words, this seems like a bluff.
It could actually be good for Tesla to be freed from his whimsical decrees (a friend of mine works there and his mercurial nature gives the company whiplash) and it could be good for him to think he has to start something new. He seems to think he can do a lot of good ai work. Maybe he can.
If history is any indicator, he seems strongest at mid-stage companies. He didn't start Tesla. He isn't doing well there right now (or Twitter, for that matter). He's too erratic for a mature company.
But he did an awesome job taking Tesla from the $M to the $B stage.
Any company at that stage would also benefit from the visibility he brings. For smaller players, any press is good press.