I will say that I am not a fan of comparing guys against trajectories of guys who became great players because it ignores the 99% of guys who had similar early career trajectories who ultimately were average or below.
I generally agree, but the comparison was about his upside as a prospect, and not who I think he's likely to turn out to be as an NBA player.
There's also the difference between whether those charts demonstrate that he's probably a good defender (they do), or whether he's likely to stick around in the league (they don't).
I'd guess most guys who have a defensive impact that tracks so cosely Robert Covington end up as good defenders, but that's not enough to mean they'll be important players. I'm sure David Nwaba and Shaq Harrison had great defensive impact numbers, but they still washed out of the league.
Terry has shown himself to be a good defender so far. He's been one of the notable hard workers, and I don't bet against those guys. That said he's a good 3pt shot away from being able to get consistent rotation minutes, but that doesn't appear to be an obstacle that will be easy for him to overcome.
I think everyone agrees that if DT doesn't improve his shooting he won't make it in the league. There are a lot of players who fall into that bucket who couldn't stick on NBA rosters.