Yep, I think you've probably got that one completely right. However from what we've been hearing, Greengrass does this quite a lot, where he leaves films, but often comes back, so don't be surprised if the director is back on board after he's sorted out his spat with Universal. He must also be aware that the only reason he gets to make movies like Green Zone and United 93 is because of his success with the Bourne sequels.
Universal's also likely to try and woo him, as they're in the middle of a potential buyout, and as Bourne is one of their biggest fanchises they probably won't want anything happening that will rock the boat too much.