If an incompetent bad actor wrote a book about web development best practices, without any knowledge of the frontend, even a layperson would quickly realize it was nonsense.

An LLM writing the same would instead create a plausible but wrong book. A layperson could be tricked by this – crucially wrong sentences would be sandwiched in between pages and pages of superficially right ones.

There is a vast ocean of difference between these two. Producing something plausible with no foreknowledge of the subject matter became possible.