I think part of what frustrates (some) GMs about Niemann, is that he is playing like Kasparov and them from before supercomputers. Modern GMs think they all know everything, since they have so much computer training, so everyone is afraid to bluff, which means a bluffer can win again. The meta has cycled back.
Post-game thoughts from the players:
Inteviewer: "Let me pull you back; so you didn't understand the position, and so you still felt like you were scared to go into a piece-up situation?"
Alireza: "Yeah so, I just trusted him. (he shrugs) I just wanted to make a move.. and play a bit more you know (laughing)"
https://www.youtube.com/watch?v=xT9orNSgk5w&t=55s
Niemann: "You have to understand, when I play the move Qg3, this is a purely pyschological move."
https://www.youtube.com/watch?v=CJZuT-_kij0&t=780s
Whether or not Niemann did cheat over the board, this shows that even top players can be susceptible to this kind of bluff.