His problem is he is a tweener, without exceptional athletic ability to make up for his height. While he has a good wingspan of 7-2 a lot of his is from his shoulders not his arm length. His standing reach was 8-10. His no step vertical when drafted was 26" and max vertical was 32.5". He weighed 265 Lbs back then in 20-21 draft, but his athletic abilities this may have been helped as his current weight is 245lbs.
A big part of his D, is being a human wall that can make people bounce off of him like Aron Baynes. Tillman really was never a block threat or the like but will have to chest up people and hold his position. So his defense is really nothing like Al Horford in terms of how he plays D.
There are going to be a lot more opportunities to get rebounds with Tatum out. I think his best overall year was 23-24.