I'm assuming it's Fultz' agent who said this, and it makes complete sense. Boston has way too much depth at the 1, 2 and 3 - and three of the top 5 prospects are PGs.
There is really only one player in the top 5 who could play minutes at the NBA PF spot, and that's Tatum. So if he got drafted by the Celtics, he'd have a very good shot of getting minutes.
If Fultz or Ball get drafted by Boston, then I seriously doubt either of those guys is stealing a starting spot from Bradley or Thomas - meaning they probably come off the bench. Most likely either the pick (or Smart) would be traded because there just aren't enough minutes to go around in Boston's perimeter.