On many Android keyboards you can press and hold various keys to get access to many of the "extra" punctuation characters and fancy "foreign" letters. I imagine the same is also true on Apple phones as well.

Indeed I am, but the point here is that some users are actually typing em-dashes outside word processors or publishing/typesetting tools (e.g., on HN)—so it's not necessarily a sign of a message written by an AI (m-dash pun intended). The poster could as well be a developer with a full-size keyboard.