There is however the flip side, where people distrust something because they believe an adversary said it. Sort of like how link spammers switched from SEO to "

There is however the flip side, where people distrust something because they believe an adversary said it. Sort of like how link spammers switched from SEO to "negative SEO" where, after Google started identifying and penalizing SEO networks, spammers started extorting people with the threat of linking to their site (thus penalizing them in search results). Blind trust and blind distrust are equally exploitable. In the end, the only winning move is not to play. If you believe an adversary said something (or "a liar" if you prefer), you ignore it entirely. You make your mind up about what you believe based on evidence, and you decide if you agree with someone based on how well their statement comports with the evidence. Naturally people will try to fabricate evidence, and even good faith evidence may be unreliable, so you'll have to do your best to access it's veracity. But what the adversary believes or appears to believe is largely immaterial.