> Nah I don't have any issue understanding, so the actual question here is why do some people find AI-generated language hard to understand?

As far as I can tell, it's only fluent readers who have trouble.

What's the test for understanding? I find that I know what all the words mean, just like in Markov chain text. In LLM text I also know what the sentences mean. In both cases I don't always know what the document means.

This is likely relative to my existing knowledge. Humans are also LLMs, and a human without prior training will be able to transfer-learn slop from a computer LLM, but a human with prior training may find the attempt at transfer-learning conflicts with what they have already learned, and reject it.

In less ML terms I think a human who reads slop and has no existing knowledge of the subject will just learn the slop by rote and confuse it for actual knowledge.