It's more the style of setting up contrasts that's the real llm tell. That they happen to use a typographic mark that most people don't know how to type is just fuel on the fire.

Em-dashes are only incidentally related to contrasting statements like that, too. My main use of them is quasi-parenthetical interpolation. It can be nice when you want more emphasis on the aside, or just to avoid using parens or commas if you started writing something that already uses them.

Anyone who types in MS word for the improved spell checker and then copies their comment to a browser will automatically get hyphens changed to em-dashes.

The fact that its not very useful for the forms of writing most people participate in nowadays--short form responses that are heavily contextual. Even longer form writing is often labored over--people use LLMs for outdated types of communication, like long-winded emails or school papers.

Idk, working in the AI space, I've started to write very succinctly and straight to the point, maybe as a counterweight to the often overly flattering, verbose forms of prose that the LLMs employ. I pay close attention to every word and try to never write more than is necessary.

You are absolutely correct.