The best expert on opining whether Niemann has been cheating or not is the player who understands the game of chess more deeply than anyone else. He understands it from the human point of view (for example: which variations can a player of X or Y level calculate accurately, which ones might seem to work but are in practice too complicated/too risky to play, which moves are absolutely unbelievable from a human because one has to find ten super moves in a row starting from a non-forcing position) as well as from the engine's point of view, due to his experience in analysing with computers. He also has recent experience in playing Niemann in a tournament.
I trust that Carlsen knows what he is doing.