People believe lies, often. That's just an undeniable fact of human nature. AIs can produce lots of plausible lies very quickly, much more quickly and at much greater scale than humans could. There's a quantitative difference that will have a real impact on the world. Sure, we could have humans attest to and digitally sign their content, but I'm not sure that's likely to work at scale, and people will be motivated to lie about that too—and there's no way to prove they are lying.
Pretty sure there will be a cost to those people eventually for believing lies. Over time, evolution will take care of it.
By which I don’t just mean survival of the fittest people / brains, but also survival of better memes (in the Dawkins sense of the word) and better approaches for bullshit detection, and diminishing of worse approaches.
