Just a guess may be Carlsen set the whole thing up. Like make a move and see if the opponent makes a move that could only be recommended by a chess engine. Do it in your preparation time and see this.
Like in your preparation you make an opening move and see what move the engine makes in response. Now do this in your live game and see if the opponent made the very same move. If they do, this confirms cheating. In this case may be that's the first he would do given he already suspects cheating?
Chess GMs at Magnus and Hans’ level wouldn't even need to cheat on every move and certainly not on any opening where something like the first 20 best moves are all memorised. These guys would only need the engine to tell with them one or two moves per game to pretty much always win.
Why did Magnus start the game and then resign? Maybe he thought it would make a statement. Maybe he needed to technically show up to the game to avoid a penalty.
One opening move can’t possibly confirm cheating since the opening moves played by the best chess engines are also the same moves played by the vast majority of human players.
But if there is no obviously desirable move, and instead a wide array of potential equally good moves, then picking the same as the computer picked gives a high probability of cheating. That being said, I find it unlikely that that's the case here.
In case of the first moves, there's definitely not a wide array of potentially equally good moves. There's a very well known set which is studied by every serious chess player.
Literally every GM (including Magnus) will have studied all the possible moves a computer will make for the first N moves to memorize what the optimal move/response is etc.
Niemann had moved one piece. There were exactly 24 possible moves he could have made. Being in line with "the engine" would have been easy.
I dont know if he is cheating. Evidence is not clear. My theory is one of two things.
1) Carlsen doesn't want to play with anyone that has ever cheated. Fair enough. But be more vocal about it.
Pet theory 2) Carlsen is on tilt. We could be seeing a shift in play style, like we have several times with poker. GTO came on the scene and suddenly the whole game changed, and those on the top many times found themselves unable to deal with how the newer players were approaching the game.