"Innocent until proven guilty" is generally used in when talking about the state and punitive decision making, not a social norm. If my three best friends accused someone of trying to rob them, I'd be comfortable believing them even if they didn't get the altercation on camera. Would the police? Depends on the country, because as you said, the robber would be (and should be) considered innocent until the state can explicitly prove their guilt.

I don't think we should live in a society where people can be imprisoned by relying on accusations alone, of course hard evidence should be required, but I don't mind living in a society where people are free to use the shared knowledge of their own community to reach epistemological conclusions. If the chess community decides to trust Magnus and his credentials without any proof, that's their prerogative, the concept of criminal innocence doesn't come into play at all here.