Grammarly also likes to suggest it in editing, and I picked up the habit of using them regularly by now. Am I an AI?
Not necessarily. Although I can't claim to be sure in your case; it just makes for interesting statistics. I'm sure that now that they are being incorporated into more writing, they are going to end up more common overall.
People will just start becoming more used to them. That's how words become more common and fade out as they gain relevancy through use. It just happens that the movement seems to be spearheaded by those tools.
Em dash isn't exclusive to english
I agree; many symbols aren't exclusive to English. However, English grammar rules often influence their usage in a way unique to English writing. Advanced punctuation typically reflects an author's confidence in their writing style and knowledge of a subject. English as a dominant language has gained a lot of influence over many instances and uses of terms, regardless of how inaccurate English was when it adopted those terms.
Your comment also doesn't directly acknowledge that it remains advanced punctuation while present in other languages, even in languages where it is more actively present, such as Russian, French, and Spanish, which primarily focus on usage in dialogue. It does not stay consistent across every language, so many translations will prefer simpler punctuation. The same languages which still don't actively have the em-dash as a key on the keyboard, requiring more than foundational knowledge to utilize.
Ergo, to use them efficiently in English writing like many new writers and authors have started to, the probability is that AI-assisted grammar tools likely have some degree of influence, not to discredit that there are many smart people here who more than likely have enough experience to have learnt and implement these things on their own.
My comment wasn't about the impossibility, but the statistics of how many hundreds of novels are uploaded every week, many who don't have a trained editor who qualifies as one of those university-level literacy students, posting novels in large quantities overall which suddenly conform to advanced English punctuation standards and grammar.
It is a general indicator for something like AI tools, which are becoming more common.
Edit: It would help if I didn't try copy-pasting from Scrivener because I prefer to put my replies into my normal writing app first, then I wouldn't post duplicate paragraphs.