If FIDE or Chess.com or whoever wanted to ban him from events for his past behavior--or players simply wanted to ostracize him by refusing to play in tournaments with him--they needed to have banned/ostracized him for that behavior. I don't think anyone would complain if Niemann were caught cheating and then permanently banned. That's what Carlsen implies he's after and it's fine.

In contrast, this is "well, you cheated in the past, but we're going to let you play, unless you play really well, in which case we'll assume you cheated". This is just not a sane way to go about it, and creates the scenario in which Niemann is playing with a sort of externally-imposed skill cap. An accusation has to come with evidence specific to that accusation, not some hazy combination of past history + unease with his play. This all sounds like a slow-motion tantrum, which Carlsen can get away with because he's Magnus Carlsen.