AI learned that from training data, which means it was used by humans writing this kind of text. At least that was true before AI started feeding on itself.

This isn't as universally true as a lot of people seem to think. I'm not defending this particular article, as I haven't even read it yet. However MS Word has been auto updating a dash to an em dash for years. Where do you think AI got it from in the first place?

It can be a marker for text that is written in places where plain text is preserved. In a written article that most likely started as a word document? Not always the case.