But to this frequency? (Note: I tried to find a study on the frequency of em dash use between GPT and em-dash prolific human authors, and failed.)

The article has on average, about one em dash per paragraph. And “paragraph” is generous given they’re 2-3 sentences in this article.

I read a lot, and I don’t recall any authors I’ve personally read using an em dash so frequently. There would be like 3 per page in the average book if human writers used them like GPT does.