The largest state that we have successfully managed to isolate from communication with the rest of the universe is on the order of 15 qubits. I think you do not appreciate just how easy it is to get information through a channel. Or rather how difficult it would be to prove that such a channel was used.

This is, however, irrelevant. Hans Niemann is a chess cheat. Allowing him to set the narrative to "I've only cheated online" is the same as allowing him to set the narrative to "I've only ever cheated while wearing green clothes".