Flatpak was started by RH employee but has been developed with significant community effort.
Flatpak uses ostree, which was originally created in GNOME for GNOME OS. And GNOME has contributors not only from RH but form Endless, Collabora, Purism and others.
Flatpak can work with OCI remotes, this is what RH more interested in. And Flathub uses only ostree. OCI remotes are used in Fedora Flatpaks repacked from fedora packages with the runtime based on fedora. But who use it anyway.
Flathub itself is independent community effort. It uses org.freedesktop.Platform based runtimes which are not based on any distro.
XDG Portals are shaped by Flathub maintainers and applications developers where RH also doesn’t play significant role.