...like many things there's likely a kernel of truth there (to paraphrase Joseph Smith). I'd also be willing to bet the bulk of it is his desire to maintain the "We Do Not Lose to BYU" culture he's so successfully cultivated at Utah. "The Speed of the Leader determines the rate of the pack". If he's all nice and congenial towards BYU it would have a trickle down effect with his players (who often have friendships with many BYU players dating back to high school etc).
I think also another element of it might be abuse he and his family get from BYU fans for being "turncoats" and "traitors". If Bronco and Holly Mendenhall didn't like the fan scrutiny they faced as the Ma and Pa of BYU football I can only guess how bad the scorn must be when you're the "Benedict Arnold and Mrs Benedict Arnold" of BYU Football.
I don't hold it against him and just see it as him doing his job. If I could go back in history and change one event though it would be that Florida didn't fire Ron Zook after the 2004 season (they lost 4 games to ranked opponents by 7 pts or less and finished 7-5 that year) and Urban has to wait another year for a high profile job to be available. Then Kyle says yes to the BYU job and starts laying the foundation for a BYU return to better days with rugged defenses, dominating linemen on both sides of the ball, multiple guys getting drafted in the NFL every year, etc.