Secondarily, I think there's a part of the discourse missing: the presence of a syntactic emdash in a sentence on the internet is not itself a strong signal of LLM-writing - but the presence of an actual emdash glyph (—) should raise some eyebrows, esp. in fora that aren't commonly authored in rich text editors (here, twitter, ...)
(option-underscore, or option-shift-dash if you prefer to think of it that way)
On iOS, you can type it by simply holding down on the "dash" button then selecting the em-dash from the list of options it presents. It may also correct double-dash to em-dash a lot of the time, not sure.
I have used the correct em-dash everywhere I can for over a decade, which amounts to nearly everywhere.