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.