Fox is not a "perfectly formed player" (we all hope for literally the opposite of this, the Rangers future basically hinges on Fox continuing to get better) but he did start his career as a great defenseman after being great in college and being great everywhere else he played before that. Whether people thought he was the 2nd coming or a "nice 2nd pair" player or a scrub doesn't really matter; he started as a Ranger as our best defenseman. People were literally comparing him to McDonagh and Leetch after what, a few weeks of playing in the NHL?
So unless your theory is that Quinn sprinkled magic powder on him during training camp, my guess is what you are seeing is simply the normal development of a player who (however underrated he was) is just really, really good. And Quinn getting credit for what coaches usually get credit for, giving him ice time and putting him in the best position to succeed.
I mean honestly, do you truly believe that the aspects of Fox's game that make him so good (his defensive reads, his awareness, his passing) would be any different if he spent the last 20 months with Torts? Or AV? Or Renney? Quinn is no more responsible for Fox being awesome than he is for Hajek being awful.
Frankly, this all just seems like a desperate attempt to justify the job of a coach who isn't doing much else.