Can you read something and tell when an author you like wrote it without seeing the attribution? I can. I think most of us could, because people use certain phrases or have a particular style. That doesn't make it bad.

AI writing isn't actually all that bad. But it has a very specific style, and there are MANY tells. I use Claude every single day, and it's immediately obvious to me when I'm reading something Claude wrote. "It's not X, it's Y" "That's the reframe" "The gap is" I could go on.

I too like using em dashes, and have been since I learned the keyboard shortcut over a decade ago. I've never been accused of posting ai-written text because of it.

AI is indeed trained on good writing, but it's also trained on bad writing. And it's post trained on specific things that are writing that aren't good or bad for reasons unrelated to style. What you're left with is a one-size-fits-all writing style. Doing math? Doing medicine? Doing anime waifu role play? It all starts from the same generic milquetoast style until you tell it not to. And to me (and apparently many other people) that's super easy to pick out.