LLMs use em- dashes because they appear in the training set. In other words, humans use them a lot. Such a weird issue to be adopted as an indicator of robot text.

Everyone unintentionally telling on themselves as not being big readers.

Em dashes are hardly ever used in modern literature. Sure, it's a tell that people aren't reading late 18th century to early 20th century literature, but that's not the same as not being big readers.

That’s an extraordinary claim.