If he's a great athlete, I haven't seen any evidence of it.
Watched him play a million times in Maine, and i don't recall him ever taking a guy to the rim and throwing down a dunk. He is rarely above the rim.
He is just an "ok" defender, lateral quickness isn't great.
Not super quick to loose balls.
Footwork isn't great getting open for a shot.
He has decent up and down speed running the court i guess.
Maybe these problems are all mental, and he isn't smart enough to get himself in a position to use this hidden athleticism - but i doubt it. I think as far as practical NBA basketball goes whatever athleticism he has is extremely mediocre at best.