I can see how it could end up as infringement, but also how one could avoid infringement. The issue would seem to be if the original author wishes to sue.
As an example, compare Brooks "The Sword of Shannara" with Tolkein's "Lord of the Rings". It is widely accepted that the former is heavily derivative for the latter; in my view essentially being an example of the above broadly similar novel.
Yet AFAIK, Tolkein's estate didn't sue Brooks. However if they had, how likely would a victory have been?
Now in the case of "Generative LLM", we may have an even closer "derivative work". However the LLM users could well get away with it if they consume a wide enough canon, and the source authors either do not learn of the derivation, or do not have the means to sue.