The original stock and vesting agreement that was part of their original compensation probably says that you have to be currently employed by OpenAI for the vesting schedule to apply. So in that case the consideration of this new agreement is that they get to keep their vesting schedule running even though they are no longer employees.
That's the case in many common/similar agreements, but the OpenAI agreement is different because it's specifically clawing back already vested equity. In this case, I think the consideration would be the company allowing transfer of the shares / allowing participation in buyback events. Otherwise until the company goes public there's no way for the employees to cash out without consent of the company.
