I've learned that when it comes to contract law, the answer to any question seeking a "can they do that?" is: depends how much they want to spend in court.

Can they write a retroactive clause into a contract? Yes. They can write _anything_ into a contract.

Is it enforceable? Maybe. And the more they're willing to spend, the more it will cost for that answer to be resolved as "no." (and even after all that spending, maybe the answer is "yes.")

Did they write this into the contracts that game devs have already signed? Dunno. Will have to ask the game devs.