This post opened a whole can of worms relating to Grav's public comments plugin. Despite being authored by "Team Grav" it hasn't been touched for going on two years and just doesn't work. It sends the notification email correctly, but does not acknowledge the comment and does not save the data.
I've taken a first look at the code, and it seems like I might just be able to wrap my head around it, but I will need hours free to do that. Hours that I do not currently have.
I could disable public comments again, and just accept Webmentions (which this post is intended to test). But although Comments are rare, some are worthwhile beyond mere affirmation, so I am loathe to do that.
P.s. It also raises again the need to fix Known's HTML-escaping problem, and makes me wonder why the comment is truncated when it gets to jeremycherfas.net -- which means looking at the templates there in more detail.