I just don't agree, not every NCAA basketball player is interested in going overseas to play professional basketball. A guy who has his best season not getting even an invite to the combine might think (rightfully so imo) that his chances of being drafted and having a shot at MAYBE getting on a roster in the NBA as a likely developmental league guy isn't that great.
The NBA is not the NFL when it comes to opportunities. The draft yields just 60 spots, and there was more than 60 declared underclassmen this year, not to mention all the graduating players and then on top of that all the overseas players that were eligible. I'm sorry, but I honestly think you're assumptions are a bit delusional.
Semantically speaking, sure he could have played professional ball this year I don't think it would have been in the US and I don't know for how long he would have played. Not everyone wants to grind overseas away from everyone and everything they know. Returning to school to play close to family is a big draw for a grad transfer especially one that may not even pursue basketball professionally.