Yes... it is an oversimplification of things. Most takes are.
But guys that spindly have traditionally had a Hell of a time maintaining weight. There is no place to put it on, and sometimes adding muscle mass means losing dexterity... which is his entire game.
Alabama was so deep, and so dominant... that most teams had no chance from the whistle. In the NFL, the margins are much tighter and the CBs are so much better. Most SEC CBs don't make it there. Some do, and are very good, but by and large, the rise in competition level is enormous.
Again... look at Iggy. Smith said he was the best CB that he faced in his time at 'Bama... and yet... half of our board thinks the guy sucks.