@phoneboy kindly shared a he said he had received.
As I suspected, it looks to me like common or garden spam, hence the scare quotes. Of course, I can't be absolutely certain without digging further into the actual URLs, which I'm not about to do, but everything about these comments screams pingbacks or trackbacks. And the solution is obviously Akismet which, to be honest, I am suprised Phoneboy has not already installed and activated.
The day may come when webmention spam is a thing, and #indieweb people have been thinking about a protocol called Vouch for that eventuality.
