That’s a stupid heuristic. I use those dashes all the time, I even have an AutoHotKey macro in Windows for en and em dashes, ellipsis and other characters.
Personally, I'm against all the new BS of no double space after a sentence and no gaps around em dashes. Not doing so removes a lot of options for those who use string searches to hunt out certain things.
But a 100% AI generated comment is bad.
And a significant (and also inevitably increasing) part of the human remnant of HN will prefer it because as far as they're concerned text generated from an AI is superior in quality to human interaction. They aren't here to interact with humans so much as consume a product.
Just teach yourself not to care. You'll never know, don't be embarrassed by it.
BTW, using stylistic features in writing to ID bots might be low-yield. Style changes are trivial to implement in any automated system, and they can be used to stylistically “blackball” particular human contributors. “I’ve heard that using ‘grey’ instead of ‘gray’ is a sure sign of a bot!” That sort of thing.