Very few of us habitually type "—" which is probably for the best. (Because then you start doing it in Word! Which....)
It's a little dictatorial to convert "--" to "—" in general but probably will make the future, and the future-you, happiest. After all, look how pretty it is when people do use the em dash on Medium!
Another possible way to steer clear of some trouble is to have some prompting or easy symbol-tools for attractive blockquotes and (as you probably do) linking... but without introducing anything wacky or proprietary. ("Learning" an input system seems antithetical to Medium.) That's brought to mind because the "embedding" a link here in Branch feels slightly goofy, yet is the simplest solution.