Again, I don't get the whole 'campaign' thing, it sounds like a big collusion.

Sure, had Carlsen won, he would maybe not have withdrawn (apparently he still felt uncomfortable with Niemann's inclusion, as did other GMs). But it's a logical fallacy to conclude that the cheating allegations are unfounded. My understanding is it simply was the straw that broke the camel's neck. Likely a false straw, but the bucket was already full at that point.