Prior to Move 6 this is a well known line with 700 hundred high level games, and Magnus (apparently) never played the line with 6.a3 before, so I find Hans's claim extremely dubious.

Still not an evidence that he cheated, but I agree that sum of signals e.g. prior cheating ban on chess.com, incoherent after game analysis without engine help and in general never being worse against 4 Super GMS and having a winning position against all of them does not paint a good picture. Something smells, but we will probably never know.

Edit: corrected move#

One minor point folks aren't mentioning that I think works in Neimann's favor - Carlsen is known for enjoying the Catalan, and the opening in the game was essentially a Nimzo/Catalan hybrid. This fact makes Neimann's supposed pre-game prep a bit more palatable. Everything else (postgame interview, past history, Twitter antics) seems to work against Neimann though.

My question is - if Neimann did cheat, how? Doesn't seem like something that can be done easily in an OTB tournament.

Or you could swallow a buzzer? The input could come from someone who watches the board from afar?

I don't know enough about those devices and metal detectors to be able to tell whether that's effective mitigation. Could one make a device small enough to be hard to detect?

Although this is pure speculation, it seems to be a more plausible and satisfactory explanation if Niemann did in fact cheat. It would also explain why Carlsen would withdraw from the tournament rather than continuing.