I think he was potential trade bait all along.
It was a win win for the jets:
If Darnold struggled in camp, then they keep probably Mccown, Teddy, and Darnold with teddy possibly starting depending on how he came back from his injury.
If Darnold looked good in camp, then trade Teddy and you still got a veteran backup in Mccown. Which is the situation they found themselves in.