You're surely right, but to me the important aspect is not only to make a package that run everywhere but also bring the mobile security scheme to desktop ie ha

You're surely right, but to me the important aspect is not only to make a package that run everywhere but also bring the mobile security scheme to desktop ie have apps that are sandboxed and run on top of a well-defined runtime with well-defined APIs. In that sense Xdg-Apps have a bigger mission that AppImage so maybe we can't really compare them