> Chatgpt uses mdashes in basically every answer, while on average humans don't

I would not be shocked if an aspect to training is bucketing "this is an example of good writing style" into a specific category. Published books - far more likely to have had an editor sprinkle in fancy stuff - may be weightier for some aspects.

My iPhone converts -- to — automatically. So does Google Docs / Gmail (althought I'm not certain if that's on their end or my Mac's auto-correct kicking in). Plenty of them out there.

> other AIs would show the same bias

Unless they've been trained not to use it, now that a bunch of non-technical people believe "emdash = AI, always".