There are three relevant types of statements: Logical arguments, independently verifiable factual claims and unverifiable factual claims.

Logical arguments stand on their own merits. Whether they're convincing or not depends on whether you can find holes in them, not on who offers them. Presenting weak arguments is low value because they're not convincing. But anonymity allows people to present strong arguments that they would otherwise be punished for presenting, not because they're untrue but because they're inconvenient.

Independently verifiable factual claims are the same. You don't have to believe the author because all they're telling you is that you can find something relevant in a particular document or clip and then you can see for yourself if it's there or not. But anonymity protects them from being punished for telling people about it.

Unverifiable factual claims are an appeal to authority, which requires you to be an authority -- it's a mechanism authorities use to lie to people -- which is incompatible with anonymity. If you anonymously claim something nobody can check then you have no credibility.

So anonymity enables people to say verifiably true things they would otherwise be punished for bringing to public attention, but is less effective for lying than saying the lies under an official identity because there is no authority from which to lend credibility to unverifiable claims.