I would imagine his performance rating so far in the US Championship would be somewhere in the 2500's.
Before round 7, I believe it was ~2588. But I meant his overall rating coming down from 2710 back to ~2500. If that happens and it remains in the 2500 range for a couple years after that, I would say we have the answer as to whether he was cheating in games from April 2021 through September 2022.

It could be that Nieman became a better player. If you get a lot better than you were, then your ratings go up. Everyone gets good ratings every now and again. Although, if you're playing on the internet, I would say that it wouldn't be too hard to cheat. In person tournaments, I think the player gets checked with a metal detector, up and down.