> because AI can trivially be told to imitate that

lowercase, maybe, but not em dashes.

You may want to take a look at the source and code sample #2 in the post - the site CSS is rendering em dashes in the source with 2 hyphens by using a custom font. Admittedly it's not the most portable solution, but speaks to (what I take as) one of the post's points that there's not a single, easy shibboleth for identifying AI writing