I guess I’m not that surprised to see people spreading the (incorrect) belief that properly used punctuation is a sign of automation. When I was much younger, I would sometimes have online conversations with strangers or acquaintances who would suddenly accuse me of trying to talk smart and using a thesaurus. People aren’t used to communication that follows writing standards appropriate for a published article. They literally think humans don’t write like this unless they are forced to, and that casual communication won’t ever look like this unless it’s cheating.
And here’s the thing. AI writing DOES sound robotic and soulless! It usually has this weird repetitiveness; doesn’t make a whole lot of sense under examination; purports to be by the same author throughout but changes tone part of the way through; references to events, people, or articles that don’t exist; and doesn’t contain any personal perspectives. Looking for a “telltale” punctuation mark as a cheat sheet–and insisting that no one uses em dashes in real life–is, frankly, just as lazy as using AI to avoid writing your own work in the first place.