> I'm not sure of the structure of what they signed before and this article doesn't get into it, but I maintain that there is no legal mechanism to tell someone "sign this or I take away something you already received". There has to be more to the story than the way I've seen it presented so far.

It's in that whole collection of tweets, but essentially the contract which you sign on joining says that if you leave and you don't sign the exit paperwork then you lose your equity. You aren't told what is in the exit paperwork at this point.

When you leave, the exit paperwork contains the non-disparagement clause.

Whether or not this is legally enforceable, no idea, IANAL. It does sound like enough of a threat that most people would just sign though