No, you didn't see anything. I've been writing like that since very long before LLMs did it, mostly because I'm considerably older than that. I'm sure if you go back to 2008, the first year that I participated in HN you'll find plenty of examples.
Perhaps you and those of your ilk are the source.
At least overrepresented in the training data...
That's not an emdash, and I don't think LLMs use dashes for emphasis that way? It's not a grammatical use.