Except if Sam and Greg have some anti-compete clauses. If they join MS, they have a nice 10 billion USD leverage against any lawsuites.
In jurisdictions where they are enforceable, yes, they generally are not limited based on the manner the working relationship terminated (since they are part of an employment contract, they might become void if there was a breach by the employer.)
I don't know, even of strictly "enforceable" I doubt we will see it enforced. And if so. I'm sure the settlement will be fairly gentle.
Edit: Actually, a quick skim of the relevant code, the only relevant exception seems to be about owners selling their ownership interest. Seemingly, since Sam doesn't own OpenAI shares, this exception would seem to not apply.
https://leginfo.legislature.ca.gov/faces/codes_displaySectio....
Your understanding is incorrect. There are some exceptions where noncompetes are allowed in California, but they mostly involve the sale or dissolution of business entities as such. There is no exception for executives, and none for people who happen to have equity stakes of any size.