I don't get this. Is football different in college?
Probably the best O line coach in the league, Jeff Stoutland of the Eagles, was 30 years in college ball before joining the Eagles in 2013 and was instantly fantastic.
Andy Reid's first staff at the Eagles came predominantly from college ball and many of them when on to be head coaches. Only 6 of that staff at previous NFL experience. Guys like Pat Shurmer, Sean McDermott, Steve Spagnuolo have only coached in college.
Football is football when it comes to position coaches.
It makes a difference with HC's because running an NFL team is fundamentally different to running a college team but for position coaches I don;t see why it matters if they came from college.
Some of these coaches will be poor and some good most likely, but it won't be because they haven't coached in the NFL before.