Since before LLMs were a thing I've thought em dashes (at least outside of fiction) have a kind of irritating, business-casual feel about them, since they are simultaneously informal but also fussy in a way. That also describes exactly what bothers me about the classic ChatGPT writing style, which is obsequiously friendly and casual but also perfectly organized and precise, and which I imagine is based on a large dataset of middlebrow American news blogs and similar sources. So I'm not surprised that the em dash has stuck out to people as such a prominent feature of its style. It definitely has its uses but I can't say I would mind at all if the controversy diminished its use in real writing.