The alleged cheating took place in an earlier game against that opponent, which Magnus played out to a loss.
Magnus played a rare opening that day, and Niemann had clearly prepared a novel defense no one had used before.
The suspicion was that someone tipped off Niemann about the opening Magnus would use.
Niemann claimed that he just randomly prepared to face that opening, which really is hard to believe.
Also, this was not the first time players had suggested Niemann was cheating.
Nieman has made several brilliant moves without showing the intensity and stress that other great chess players would need to find an original over the board move.
Interesting info about Nieman. Thanks for that.