IANAL but I think if they knew the statements were false (or with reckless disregard for the truth) then it would be slander. This is the "actual malice" standard. But in this case it seems absurdly unlikely that Niemann could prove this.
Is this guy enough of a public figure to require the actual malice standard? Because for a private citizen, you would only have to prove that the accusations were wrong, assuming the topic is also not considered to be of public concern.
But maybe chess championships and champions are a big enough deal to be considered public.
In this case everyone mentioned in the suit public figure within the worldwide chess playing community which would suffice. Also this event has had international coverage from multiple sources which would be presented as evidence that they are indeed public figures if there was any doubt.
