I don’t dispute that, but the point remains. One game is not an adequate body of evidence, regardless of the circumstances.
OTOH, this is a cutthroat "business" and when you do get a chance to prove yourself, you need to make the most of it as you don't know if you'll get another. Especially at a position like QB where only one guy can play.
If you do have an opportunity to pursue a guy who has proven he can get it done at the highest level, you'd be ill-advised to pass up on it simply because you have 2 other guys in the program.