The argument isn't 'HC's with NFL only experience do great when they move to the college level' so no need to cite guys who have done it.
The argument is that HC's with NFL only experience do poor when they move to the college level. That is the side who needs to cite someone, and I say it's a myth because for the most part they can't. Lane Kiffin is one they'll throw out, but truthfully he has been WAY more successful as a college HC than he was in his 20 games as an NFL HC.
So if someone is a terrible NFL coach but when they go to college they jump up to being average to slightly above average, does that support that NFL only experience HC's don't do well at college level? Or does it better support that even really shitty NFL HC's do much better at college, while good or even average NFL HC,s like Carroll, Saban, Mora do pretty darn well in college?
The deck is kinda stacked one way here. First you only get to work with NFL HC's that were never college HC's (or even spent 15-20 yrs at college level as coordinator, etc..). So now out of that limited pool, how many went down to coach at the college level? Of them, how many did worse at the college level than they did in NFL?