There are a ton of people like that, but the LLM house style also exists because a ton of people write that way too.
The people falsely accused because they've used em-dashes for 20 years aren't the ones that were functionally illiterate before.
I think em-dashes were uncommon mainly because they're not always convenient to type.
I don't think there's any definitive way to check, but for me one of the biggest tells that a long piece of writing was LLM generated is that it will hardly say anything given how many words are in it.
(well that and the "it's not just x, it's y!" pattern they seem to love)
It's not just patterns like "not just X, but Y", but also deeper patterns and a kind of narrative cadence. Sure it's also mimicking something real, but usually it's a mismatch between the insightfulness of the content and the quality of the delivery. It feels like chewing on empty calories, it's missing the intentionality and the edge of being human. I guess you need to read a lot of LLM output to get a feel for this beyond the surface level pattern matching.