The one tool companies have at their disposal in CA is the prohibition of using your old company's trade secrets in your new job. Mostly that means you can't take customer lists with you, but you also can't use confidential information to lure away former colleagues.
What that means in practice isn't completely clear to me, though it may just be that you can't take the org chart.
Very high senior management have been held to a non-compete if it is very specific and time-limited.
Self-driving is quite specific right now. So, if they limited it to a year, it would probably work.
However, the issue here is things he did while still working at Tesla, not after.