The problem with this statement is that it assumes that "fact-checkers" are a single entity when that is not the case. The answer to "who fact-checks the fact-

The problem with this statement is that it assumes that "fact-checkers" are a single entity when that is not the case. The answer to "who fact-checks the fact-checkers" is "the fact checkers". And we can use consensus, reputation, and checks against known facts to validate the checks themselves. If group A, group B, and group C all agree and group D disagrees, then we can say with relative confidence that D is probably wrong in this case. It is safe to go with the consensus. However, if A, B, C are notoriously unreliable and have a history of distorting claims, then it is beneficial to see why D disagrees with all three. Or even not use A, B, and C as sources. We can also judge a fact-checker's veracity by how they evaluate a claim. Do they provide sources for their claim. It's one thing to say "This will murder 40 people a minute" and quite another to say "This claim disagrees with a recent study by the CDC [link to study] that says it will only murder 39 people a minute". The only reason to cast doubt on the process or activity of fact-checking is if you don't want people checking your facts.