Maybe they have statistics that indicate that their larges user group by now isn't capable of using markdown? Most non-techies are not aware of markdown.
Maybe they have, maybe they don't. Or maybe they can pull some out of their behinds on the spot when asked (which I suspect is the most likely case).
If non-techies can be taught how threading or bunch of other Slack idiosyncrasies work, they can as easily be taught that surrounding text with characters like *, /, ~, _ and ` changes formatting. It's a trivial concept. Reddit, or countless Internet forums before, never had a problem with teaching that to non-techies.
