> If Magnus destroys Niemann, he'll be able to pretty handily claim that Niemann can't win without communication to computers. If Niemann is competitive, or wins, it will be hard for Carlsen to make any more such claims.

I don't think that's very conclusive.

The thing is, there's still an element of luck[1] in chess. A lower-rated player can beat a higher-rated player, and a higher-rated player can dominate an even match.

[1] luck, not chance.