This is more what I was getting at. I think it's the system, but credit to Heupel because it's his system. But I don't think these QBs are getting "fixed". Milton still overthrew the ball. Nico probably still doesn't check down. Joey still threw INTs. But the system worked for them. Or at least good enough.
Not trying to be negative about Heupel. His offense is dynamic and exciting. Just noting that he's not asking guys to change. They are who they are. But they succeed, more or less, within the confines of his system.