If you install it from the debian repo (and likely any other distribution repo), JOSM is already registered as an application for geo: links out of the box.
I am on Fedora, and I no longer remember why I thought the flatpak version is not optimal. Anyway, yes, it is associated even in my .desktop file I have for JOSM somewhere, but it does not do much - opening a link just opens a new JOSM instance, and does not even load any map, so it is quite useless (does it work differently on your system?). What I documented in the wiki interacts with your running instance and zooms where the photo was taken.
That way I exactly see where the photo was taken. I then usually delete it right away. And anyway, even if not - the node would be empty, so Validation would catch it.