> Plenty of people have known how to use em dashes

Plenty? I don't think it's that many. And knowing how to use them, doesn't mean they would actually use them. In my experience, em dashes are mainly found in academic literature. Chances that an em dash in a new text comes from an LLM are pretty high.

Your use of it is dubious, I think. Commas would serve equally well --or better-- since it's a simple conjunction, not an interjection.

Markdown is ubiquitous in the kinds of places Hacker News visitors also tend to visit, like GitHub, various ticketing and project management systems like Jira, note takers like Obsidian and Notion (or an approximation of Markdown).

I've used `--` for years in place of an em-dash, which markdown renders as an actual em-dash.