I'm not a legal scholar, so I'm not qualified or interested in arguing about whether Cliff Notes is fair use. But I do care about how people behave, and I'm pretty sure that Cliff Notes and LLMs lead to fewer books being purchased, which makes it harder for writers to do what they do.

In the case of Cliff Notes, it probably matters less because because the authors of 19th century books in your English 101 class are long dead and buried. But for authors of newer technical material, yes, I think LLMs will make it harder for those people to be able to afford to spend the time thinking, writing, and sharing their expertise.