I was surprised to learn from your comment that em dashes were left out of ASCII, because I thought I've been using them extensively in my writing. Perhaps I'm just relying heavily on the hyphen key. I mention that because it's likely instances of true em dash use (e.g. in the high-quality text you cite) and hyphen usage by people like me are close enough together in a vector space that the general pattern of a little horizontal line in the middle of a sentence is perceived as a common writing style by the LLMs.
I find myself constantly editing my natural writing style to sound less like an AI so this discussion of em dash use is a sore spot. Personally I think many people overrate their ability to recognize AI-generated copy without a good feedback loop of their own false positives (or false negatives for that matter).
British Airways had a famous advertising campaign that did something similar. If I remember correctly they used a digital billboard somewhere in London that showed a little boy pointing to the sky. The text refreshed throughout the day to display the details of the flight passing overhead.
I find myself constantly editing my natural writing style to sound less like an AI so this discussion of em dash use is a sore spot. Personally I think many people overrate their ability to recognize AI-generated copy without a good feedback loop of their own false positives (or false negatives for that matter).