Its about the actual character - if it's a minus sign, easily accessible and not frequntly autocorrected to a true em dash - then its likely human. I'ts when it's the unicode character for an em dash that i start going "hmm"
Mobile keyboards often make the em-dash (and en-dash) easily accessible. Software that does typographic substitutions including contextual substitutions with the em-dash is common (Word does it, there are browser extensions that do it, etc.), on many platforms it is fairly trivial to program your keyboard to make any Unicode symbol readily accessible.
Us habitual users of em dashes have no trouble typing them, and don’t think that emulating it with hyphen-minus is adequate. The latter, by the way, is also different typographically from an actual minus sign.
