The suspicions of Chess.com about Neimann’s cheating in online games (which also raise his ELO score) have to do with the use of multiple monitors while playing the games.
No issues with the use of multiple monitors - many GMs routinely play simultaneous games.
But they stated that his head movements checking other monitors coincided with the playing of the “best move”, which is where they came up with the suspicion.
No concrete suspicions since they were not videotaping his online setup, but it fit the pattern of their cheat detection algorithms.
Neimann may be clean as the driven snow but enough GMs feel that he is playing outside of the boundaries of chess, to make them feel uncomfortable.
Which they do not sense from any of the new young GMs from India, for instance.
I don't believe Niemann played any online games under FIDE.
I don't know about an accusation of two monitors, but they can detect when the user opens another tab on the same computer. It is called toggling. It raises a different screen on the same computer. They can correlate when tabs open in relation to play improvement.
Niemann may be cheating over the board but, if so, he has not been caught. Chess.com found him to be clean online and over the board since 2020. What Chess.com published in the Wall Street Journal will not be considered by FIDE because FIDE must meet scientific and legal standards, and Chess.com refuses to submit its system to peer review. Until they have their system verified, it will never meet the legal and scientific standards required by FIDE.
Until someone comes up with actual proof, there is no justification to drop out of round robin tournaments or throw games in tournaments.