You just named a few head coaches that have been primarily NFL that have been successful going to the college ranks...but can you name any who fit that mold then completely failed at the college ranks?
Yes there are plenty of coaches who struggled in the NFL, but did good at the college level...this only bolsters the belief it is a myth.
All the successful NFL head coaches you can think of who also did college, did well at it -AND- a good number of them who bombed as NFL head coaches did very well at the college level.
That is why I say "it's a myth" that, the success rate of NFL head coaches who move to college, is fairly low.
In fact it's actually the opposite, the success rate of NFL head coaches who move to college is fairly high. Not enough quantity to make it a gold standard (as in just hire an NFL head coach and you're good) but of the few who have done it the success rate IS fairly high.