This absolutely doesn't look like AI writing and this is a pretty weird signal to use.

I have a much better theory: as it happens, Substack automatically converts "--" to em-dash as you type. I'm guessing the author knew that and wanted to use this feature, but composed the article in another editor and then copied-and-pasted the whole thing. Because of how Substack implements the feature, the substitution doesn't actually kick in on Ctrl-V, so all these "--" remained in place.