I just learned a bit about Specialized's "Plug and Play Fender" that apparently fits my bike by way of metal rod into the leftmost hole of the first photo, secured by the set screw in the second photo.
This made me insatiably curious as to what the other threaded hole is for. The marketing materials I read before buying the diverge said that only the aluminum model had eyelets for a rack, and in this carbon model there's no eyelet at the top of the seat stays for a rack - just that one lower eyelet. It does not appear to be 1/4-20 thread, for what it's worth.
What could it be for?