Bread, it's called risk/reward. He was projected top 5 before he got flagged with a bad back and got drafted at 21, very low risk move for Boston.
No, it's called exaggeration. If no one believed he would survive the NBA, he have been drafted. The Celtics doctors and Ainge believed his back wasn't as bad as advertised. Gronkowski fell to the Pats for the same reason, and his back hasn't been a huge issue throughout his career.
We'll see if Sullinger continues to have back troubles, it's silly to not consider him at all in the future of the Celtics as it is right now.