Yes. Niemann has admitted to cheating in the past, and has apparently been banned from some past events for cheating. So Carlsen can safely relate to the public that he believes Niemann to be a "cheater". But to say for a fact that Niemann cheated in a specific match, he'd be communicating a statement of fact. If that statement is false, or could colorably be argued as false, then Niemann can take him to court for defamation, and even if Carlsen prevailed, it would still be painful and expensive.

Remember that statements of opinions, including opinions that are analyses of previously disclosed facts, are protected from defamation claims. Defamation can only consist of a damaging false statement of fact, or the allegation that you're aware of specific undisclosed facts like that to support your opinion.