He does not have to be good enough to make the pros. He just needs to think he is good enough. Given he left high school a year early, I could see him leaving college early. He obviously believes in himself enough to step up to higher competition ahead of schedule.
This.
Great athletes have a lot of confidence. They also surround themselves with yes-men (and yes-women) who tell them how wonderful they are.
As for the OP ...
It is highly unlikely that he makes more than half of his treys again, but it's certainly not impossible. Teams that make him the focal point of their defense can stop him, so it will be interesting to see how opposing coaches play him.
If he stays three more seasons, stays healthy and doesn't get Haanied, he should get into the top-5. But him averaging 17-18 ppg for that 2018-19 team, with all the other options out there - including Grimes, of course! - will not be easy.