You can't possibly know this to be true. You can believe it, you can assume it, and you can say it. But it's nothing but your opinion.
Had he put on 10-15 pounds, returned to show more aggression, more consistency, and better rebounding, he certainly could have improved into a first rounder. He could have easily averaged 20 and 8 on this team and shown the ability to be a consistent scoring threat.
It's possible that would not have been enough to get him into the first round, were it not, Australia, Israel, and Europe will always be viable options. However, it's not unimaginable for a senior to play himself into the first round (see: Napier, Shabazz).
If you can't understand the HUGE difference between being pick #30, and pick #31 (literally a few million guaranteed dollars) then you'll never get the point
@Jerry1714 is making.