"I would just needed to cheat one or two times during a match, and I would not even need to be given moves, just the answer on which move was way better, or here there is a possibility of winning and here you need to be more careful. That is all I would need in order to be almost invincible."
Even just 1 bit - an indication to be careful - would be enough to boost the strength of a GM. An accomplice coughing in the background to let you know there's something to watch for. For a strong player - and there's no doubt that Niemann is a strong player, the question is just how strong - that's all they need to avoid making mistakes. GMs can solve insanely hard puzzles, because they know it's a puzzle and has a specific solution. Same thing with 1 bit of info.
Of course, realistically they could simply use Morse code instead of "bits" and transmit two squares (just 4 Morse "letters").