They're not checking facts, they're just enforcing the mainstream view. The mainstream view is often correct, but sometimes it isn't, and then the fact-checkers are just horribly wrong, and suppressing actual debate on the merits of an issue.
https://en.wikipedia.org/wiki/COVID-19_lab_leak_theory
“Some scientists agree that the possibility of a lab leak should be examined as part of the ongoing investigations into the origin of COVID-19, though they have expressed concerns about the risks of politicization.”
You argue that blocking debate is a righteous act because the theory's "very existence harms public health". I'd counter that the gagging of a concerned public breeds distrust that is vastly more harmful than allowing debate would have been.