Getting a Top 15 grade, and actually getting drafted in the Top 15 are 2 different things.
The teams that need a QB badly are also the teams that consistently lose, and most likely can not protect their QB. He is too much of a risk, for those teams.
There are probably lots of NFL GMs that would tell CNS "sure, he is a top 15 pick", and hope he falls into their lap, at lower price. They would also be teams that do not need a QB ASAP, who can not only give him the time to learn, but rehab as well. His chances of having a longer career, along with that second contract payday, are better if he is not drafted in the Top 15.
About the only thing certain is he will not be picked in the first 5, this year.