It might be more fun to see users who’s emdash usage increased after the release.
Maybe the HN crowd is the wrong group for such statistics, a higher percentage here probably knows how to use their keyboard and OS.
I think they meant after the release of ChatGPT. If someone never used them before and now uses them all the time it might indicate that they're using ChatGPT... or it might just mean that they learned how to use them after widespread discussions about it.
I use em-dashes now more than ever — mostly just to mess with people.