As I've said before, the abilities of the ones with ChatGPT assist don't seem to be known, as they started off with help. The assumption is that they might on average be the same as the others, but we don't know that. A simple fix to start is to offer the LLM help in the middle of the test for them, see how the curve of ability goes up and down, and compare before and after.
Not answering the rest, I was more interested in the study and what it was claiming and not arguing about missing context of commentary. I think the study may be right in suggesting people today have more trouble understanding things than they used to, but it's far deeper and older than LLMs. They are just the latest part of the bigger problem.