Comment by grey-area

So we’ve established the default is a hyphen, not an em dash.

You can certainly select an em dash but most don’t know what it means and don’t use it.

It’s certainly not infallible proof but multiple uses of it in comments online (vs published material or newspapers) are very unusual, so I think it’s an interesting indicator. I completely agree it is common in some texts, usually ones from publishing houses with style guides but also people who know about writing or typography.