(There's zero evidence it actually was an anal device; no one knows how he might've cheated if he indeed cheated. It just became a big meme.)
I do think Niemann may have cheated in OTB chess before. But not at the Sinquefield Cup, I just don't see it. And the only evidence is insinuations from a sore loser who's well known for throwing tantrums when he doesn't get his way.
The buttplug stuff was really just a joke from reddit, no one's seriously suggested anyone is actually cheating using a buttplug. Doesn't even make sense as a way of cheating, really. It would be extremely obvious.
The idea that Carlsen can't stand losing is a theory from people who don't know about chess. His lose rate is about 15%. All GMs lose games all the time. It's keeping the win rate as high as possible that makes them the best.
The notion that any GM would get so upset simply about losing a single game is just nonsense.
Specifically, move 19. Qd3! [2], appears to hang the knight. When asked how he'd respond if his opponent simply took the knight (19... dxc4), he says he is "completely winning", then proceeds to give some moves that lead to a flat-out losing position. (Eg: Any GM or sufficiently strong engine agrees that r2q1rk1/1pbn1p2/2p1b1pB/p3P3/P1p2P2/2P3QP/1P4P1/3RRBK1 b - - 0 1 is lost for White.)
To be fair, he's been playing incredibly well recently, and no other evidence of cheating came out, so I'm inclined to believe it was just a "bad interview."
[1] https://www.youtube.com/watch?v=rI9jAU0jhJU
[2] https://www.chess.com/events/2022-sinquefield-cup/04/Niemann...
And GMs have had plenty of bad reactions to losses throughout the history of chess. It definitely happens. Chess is an emotionally taxing game. You spend many hours of exhausting effort at the board only to lose due to a momentary brainfart. It's infuriating. GMs are humans too. Hell, fairly recently there was an incident when GM Christopher Yoo punched a photographer after losing a game at a tournament in St Louis.