No one is asking that we reject all prose with emdash. Not all emdash-users are LLMs, but many LLMs are profligate emdash-users, so adjust your priors accordingly.

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, ...)

Before LLMs, the em-dash glyph was a decent tell simply that... the author was using a Mac, because it's a simple and easy-to-remember (or even guess!) key-combo on there. Not that you can't type it on other keyboards, but the Mac one for whatever reason had a combo of users-who-wanted-to-type-it and layout-that-makes-it-easy that resulted in a high proportion of correct em-dash employers being Mac users.

(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.