I just don't get this love affair people have for Timelord. He's an athletic big man which is great but when I see him play he's often out of position on defense (sometimes just not paying attention or he's looking for a block), doesn't block out on rebounds so he's not controlling the defensive boards (to be fair, few players on our team block out effectively), doesn't get many offensive rebounds (Kanter is easily our most effective player at positioning himself for an O board) and his offense is pretty limited to rimrunning.
Actually, this is a misconception some folks have about the Celtics. This Celtics team has been the 3rd best team in the NBA at boxing-out.
Box-outs per 48 min:
Miami - 9.6
Toronto - 9.1
Boston - 8.8
LAC - 8.8
NYK - 8.7
MIL - 8.6
Boston was also 3rd in 2018-19. So Boston, as a team, actually DOES box-out pretty aggressively.
On Robert Williams in particular, it is fair to say that within this team he has been sort of middle-of the-pack at boxing out, at 3.7 box-outs per 36. The guys who really box-out aggressively are Enes (7.5), Tacko (7.5), Vincent (6.9) & Theis (6.2). So relative to the other bigs, yeah, Robert isn't boxing out as aggressively. Even Grant, who ranks just above Robert on the team at it, is comfortably ahead at 5.9 box-outs per 36.
Now, all that said, most of the difference is on defense. And if what Smart has said holds water then it is possible that the RW3 is at center and the perimeter guys around him are playing more aggressive on the edge, that RW3 then has more back-stop shot-contention responsibility. If you've ever played center in a 1-4 zone you know that boxing out is lower on your priority list than when you are sharing the paint with another big.
But ... then I look at Tacko's numbers. Because when Tacko is on the floor - that is definitely what happens and he's still doing a lot of boxing out.
So while I am not as down on Robert as you seem, I do think that this particular criticism may have some merit.
I do still think he has absurd potential, though. His hops are unreal and he at times seems to have a great natural feel for the floor. His passing on offense is very good and his ability to cover space not only has him contesting a lot of shots per minute, but also have a significant impact on the efficiency of those shots. I'm not ready to give up on him just yet.
All numbers courtesy of stats.nba.com.