> Decide whether it's accurate, insightful, worth thinking about and researching further, etc. based on its substance

This is the part the original human poster is assumed to have screened as a first step, not the audience, particularly if the audience is unfamiliar with the subject (such as a guide, etc).

I literally came across a guide online from a user who wasn't a spammer, who disclaimed they haven't even read the very guide they posted as an article on their website, as it was LLM generated. At least that user put up a disclaimer but why would I trust such a guide, given my and others' extremely inconsistent experience with the veracity of LLM output and as someone coming to the guide to learn (ie: not a domain expert)? Overwhelmingly other users don't put up such disclaimers so we don't even get to know whether they've vetted anything.

Trust is the key thing. To continually erode reader trust means you're putting the burden at every step on the reader. Sure, one should always apply critical thinking to even human output but there is an implicit, baseline assumption that with human output they're at least familiar with what they've output (whether they're lying or telling the truth or ignorant but honest). LLMs meanwhile handle ground truths in a flaky way, such as when they'll hallucinate quotes from even articles they claim to have read and cited. And the most common models users are using are the cheapest/free ones anyway, only compounding the accuracy issues.

Imagine you went to a library assuming authors, publishers and library staff have done some minimum due diligence only to find the library is being replaced rapidly with books that no one in the chain has read.

No one can be a domain expert in every single thing they encounter, which is why we place trust in others to varying degrees to fill in the gaps based on their experience and knowledge, even if you're a dyed in the wool skeptic. When increasingly what we encounter isn't being vetted as a basic first step then it's a waste of time and rude to the audience, which only decreases peoples' tolerance for bullshit and increases cynicism (which we could use less of).