LLM speak is of "higher" "quality" to a superficial inspection because it uses big words in a grammatically correct way.
It is worse because it fails to communicate information well. When I say it can barely speak English, I mean that it produces sentences which are incomprehensible on the first few reads. I have never found the median literate native speaker to be so difficult to understand, partly because they use a limited vocabulary and avoid waffle.
It can barely speak English in a new and exciting way, one unlike all the previous humans who could barely speak English.
I don't think the claim will bear out that LLM-generated text fares more poorly on comprehensibility tests than those of the median literate speaker. I understand the aesthetic arguments against it, and largely agree, but the technical argument you're making here is almost certainly and starkly false.
Most people write quite badly!
I would describe PIAAC literacy levels 1 and 2 as barely able to comprehend English, and someone who wrote at a corresponding level as barely able to write. They're close to the line of illiteracy, particularly PIAAC 1.
I believe the median person who comprehends English to PIAAC level 3 literacy or above, and writes accordingly, would be more comprehensible than an LLM, if we controlled for correctness and for their own understanding of the subject matter.