I've heard this a lot, so it must be true, unless it isn't. If he isn't good at making quick reads and throws, then why was he able to handle blitzes so well and do such fantastic work at the end of some games in high-pressure situations?
If you've done much coaching, you've probably known kids who can do amazing things in practice, or on the spur of the moment, or if you surprise them, but in game situations when they've had a lot of time to anticipate, their talent seems to disappear. I always chalk it up to a lack of confidence, or the yips. Conversely there are kids who seem perfectly ordinary until game day, then they transcend their own ability level and elevate themselves above their peers.
I've encountered this in baseball and boxing for years, and I still haven't figured it out.