There may be some truth to that, but I doubt they would have kept him for as long as they did, if he didn't know any plays or routes. The problem is mathematical; there are only so many touches to go around and that is particularly the case for a team that likes to play ball control / ground and pound. Add to that point that he was living in the Goats shadow, and it is somewhat understandable why the Cowboys do not feature their second tight end often.
But with Rico, given his height and knack for going and getting rebounds, he is ideal in red zone packages and he has proven he has ability with the ball in his hands after the catch. If he were to go somewhere like the Saints, Patriots, or other team that uses TE's, he would be a monster. I'm not so confident about that with the Cowboys; they've expressed interest in getting their other TE's involved in the past, but they never actually do it.