I will defer my opinion to that of someone who knows more about chess than me. According to Grand-master Anatoly Karpov’s analysis of the game:

“Carlsen surprisingly played the opening so badly with white that he automatically got into a worse position. Then he showed a strange inability to cope with the difficult situation that arose on the board. Comments that White lost without chances are complete nonsense.

Yes, he played badly, his position was worse, but he didn’t have to lose: if he hadn’t sacrificed a pawn there would have been nothing terrible for him in that position. With more careful play he could have made a draw.”

Maybe Niemann cheated, maybe he didn’t. But the true is that the reason Carlsen quit was because he got flustered. He could’ve, and as a matter of fact had previously, kicked Niemann’s ass with skill. He could’ve made this a draw, keep playing the tournament and still dominate. But he played one bad game and had to throw a hissy fit of historical proportions.