I have almost no knowledge of chess, but is the argument that Carlsen played so mysteriously bad that people began to claim it was cheating, and when Carlsen withdrew, that greatly confirmed such suspicions?
Like as if Michigan lost to Appalachian State - which, while embarrassing, is still plausible - but then Michigan suddenly started to act shifty and weird?
The salient point is: did Niemann consistently find the best or near-best moves (as specified by Stockfish*) throughout the game? If so, this is very suspicious because GM’s are only human, and shouldn’t play like a computer.
*Chess software widely used to analyze games. And used by cheaters for their moves.
No. Carlsen lost a game he wasn’t expected to lose, then withdrew from the tournament with a cryptic message that many have interpreted as an allegation that his opponent was cheating (by somehow receiving and using the best moves as determined by a computer).
As I understand it, a key part of the circumstantial evidence against Niemann is that when his games were being broadcast live, he played very well (including against Carlsen), whereas when the organisers put a 15min time delay on the broadcast (which would make it virtually impossible for an accomplice to put the position into a computer and then secretly communicate the best move to Niemann in a timely fashion), he played relatively badly.
I haven’t read/watched a huge amount about this case, but I think 2 things are clear:
It can never be conclusively proved that Niemann cheated in this case (barring a confession);
Chess tournament organisers should take reasonable steps to prevent cheating (such as the time delay mentioned above) for the good of the integrity of the game.
No good cheater is going to follow Stockfish moves for too long — it’s too much a tell. This is my understanding from reading and watching a ton about this controversy. It seems to be that they both didn’t play particularly well. I thought Magnus knew something we didn’t, based on this being very out of character for him, but after going back and forth about this, I don’t see any reason to suspect Hans cheated, and Magnus displayed poor form in dropping out of a round robin tournament (which messes up the rest of the players.)
Niemann had declared he’d been prepared to counter the world champion’s moves because that morning he happened to rewatch a 2018 game between Carlsen and Wesley So in London, where Carlsen had employed a similar opening move to the one he used against Niemann. But as chess fans pointed out, Carlsen and So never played each other in London that year - though they did play in a 2019 match in India, where Carlsen deployed a different version of the move in question. However, as Defector reported, Nakamura and So himself argued that the  game’s different structure nullified comparisons between the two.”