As you say, About on that map links to this uMap user page of Michael Courtial.
It might be a coincidence, but there exists the OSM user with same name Michael Courtial. So I’d try clicking there and choosing Send Message to them if they are indeed the same person, and if so, can they update the map.
Failing that, one might try contacting uMap.openstreetmap.fr and ask theming if they can help you get in contact with the user that created the map (preferably composing a message for the map creator explaining what you want, and asking them if they’d kindly forward it – leaving it to the map author to choose if they want to get back to you).
