The only way I can see this being a valid contract is if the equity grant that they get to keep is a new grant offered the time of signing the exit contract. Any vested equity given as compensation for work could not then be offered again as consideration for signing a new agreement. Maybe the agreement is "we will accelerate vesting of your unvested equity if you sign this new agreement"? If that's the case then it…
It's not. The earlier tweets explain: the initial agreement says the employee must sign a "general release" or forfeit the equity, and then the general release they are asked to sign includes a lifetime no-criticism clause.
They're really lending employees equity, subject to the company's later feelings as to whether the employee should be allowed to keep or sell it.