I think the Bills FO looked at 2024 as a rebuilding year. They drafted Coleman hoping he would grow into the role. Separation is a skill that can be learned. Coleman does have great body control, which is something that should be able to translate into doing the little things to shake off a player. Now he still has to learn that skill, and obviously hasn't mastered it yet.
Davante Adams couldn't separate much early in his career either. At some point (I think it was after year 2) he decided he was going to buckle down and learn it, and he did.
Lots of guys MIGHT be able to learn how to separate, but only some of them do. Just like lots of big strong armed guys with accuracy problems MIGHT learn how to be more accurate. Josh Allen did learn it, but not in his rookie year. Wil Levis has not learned it, and neither has Anthony Richardson.
The potential is there for Coleman. But he has not realized it yet.