I think this is misinformation, a red herring or stalking horse. It's also a bit anti-intellectual, as if people haven't been using em dashes for forever, long before LLMs existed.
I'm fairly confident it's true. I just asked ChatGPT to generate a 1000 words comment for HN[0] and it used 15 em dashes. Now scroll through HN comments and count how many em dashes you encounter[1]. You can go multiple pages without encountering a single one.
[0] https://chatgpt.com/share/6836f23d-9b58-800b-8cea-0a86a58076...