Winston and Marriotts weren't that long ago. Anyway, I agree, but didn't expect him to come along this fast. That being said, it's one preseason game. Let's give it a little time...
Competition level doesn't affect pure accuracy, velocity, and delivery though, those are independent traits and they all looked very good, that's my reason for thinking he's gonna be legit.
Now of course, higher stakes could lead to a breakdown in form, his release gets clunky, he gets nervous and makes poor throws or holds the ball too long, etc. but if a guy isn't doing that in his first game under NFL lights then he probably won't get rattled in the regular season either. If hes able to stay true to the form he showed in that game, then no matter the competition he should be solid at least