If the game was live and face to face, I don’t see how he could have cheated, I doubt he was wearing an earpiece or had someone signaling him the next move, I guess it’s possible but highly unlikely IMO.
What Niemann might have done is practice against a computer engine that could look at Carlsen’s moves over some number of games he’s played and offer counter moves that Niemann employed.
I’m just speculating.
Vibrating anal beads.....
The claim is that there is a device inserted anally (no, I am not kidding) which involved a series of buzzes (please no sexual jokes) signals the person on moves. Obviously there is a code involved.
He had some chess pieces up his sleeve....