Cheating between grandmasters in an over-the-board game doesn't really need a system requiring electronics.
Kasparov once said many years ago, that all he would need to get an edge is to know that there is a devastating move to find. Once you know that such a move exists, you can concentrate your time (chess at this level is really a game of time management) on the key move. The grandmaster can find the solution themselves from there.
Communicating that type of information is very cheap and easy. It's just one bit of information - normal move vs critical move. It could be done very effectively by someone sat in the audience holding a water bottle. Hold it in the left hand normally, and then hold it in the right hand to indicate the critical move.
It sounds ridiculous but this is something that has worried grandmasters for decades. In the 1978 World Championship, Korchnoi objected to tubs of yogurt being bought out for his opponent Karpov. It was eventually agreed that he could have the same flavoured yogurt bought out at the same time every match, thereby eliminating any information being transmitted by flavour or time.
Once you realise the possibilities of signalling in this way, it's very easy to see how a player can be driven to distraction or paranoia; and I suppose in extreme cases, madness. "If it's not the tub of yogurt or the bottle of water, maybe it's the flicking of the lights, or the cough in the background or the sound of aeroplanes flying overhead..." The possibilities are almost endless.
So while there's no evidence of it happening in Niemann vs Carlsen, cheating (for a limited definition of cheating) in live events is not as difficult as it first appears.
My own opinion is that Carlsen is simply paranoid but given Niemann's admitted history, it's not completely unjustified.