Yes, that´s true but that is not exactly what I meant:
Firefox does not have a repository on www.github.com/flathub as it builds its flatpak package on their (mozilla) infrastructure and pushes it directly on the flathub servers
OBS Studio builds their flatpak packages with their CI infrastructure in their repository and has therefore archived their repository on flathub
Gnome and KDE on the other side develop their apps in their repositories and infrastructure (and are actually building already flatpak packages as part of their routine tests) and maintain an additional flatpak manifest on www.github.com/flathub.
So, the point I just want to understand is why Gnome and KDE maintain two (and probably identical) flatpak manifests in two separate repositories when one flatpak manifest in one repository would be sufficient (just as Firefox and OBS studio does it).
They are not identical—you could have simply checked yourself.
The manifest for Flathub releases is for stable releases; it depends on the stable GNOME run time, and typically uses release archives or release tags.
The manifest in the application’s repositories on gitlab.gnome.org are used for development purposes; they link to the nightly run time, and they are either used for the CI pipeline, or published on the nightly GNOME repository.