> Him showing up for a single play of a rematch and then quitting after the first move seems mostly like a tantrum.

I'm not a chess expert at all but, from my understanding, he is known for some unconventional play. I could see Magnus having done a calculation ahead of time of a very unconventional opening and, seeing the counter being the exact optimal choice, being convinced of cheating.

All that I really know is he has a lot of credibility and the person he's accusing does not. I think it's a big story because it's dramatic no matter how it turns out.