I don't think it's a myth. I haven't studied it and haven't compiled any stats, so I'm not trying to argue or flame as I'm going on empirical evidence. Can you name more than a few coaches that have been primarily NFL coaches that have been successful going to the college ranks? Admittedly, there are fewer examples of NFL to college as there are college to pros, but off the top of my head, other than Pete Carroll, Jim Mora, perhaps Bill O'Brien, I can't think of many.
There are some successes going from college to pros. Jimmy Johnson, Harbaugh, Tom Coughlin, and going way back, Don Coryell and Paul Brown. But I would dare say that the flops have been more the norm. Devine, Holtz, Saban, Spurrier, Petrino, Chip Kelly, Butch Davis, Bud Wilkinson, Frank Kush, Dick McPherson. That's without thinking very long or researching it.
Like I wrote, I'm going off of empirical observation. If you can show that there have been more successes than failures then I'll admit it is a myth.