Be aware the sandbox of flatpak is not safe for web browsers, specially firefox based browsers:
https://seirdy.one/notes/2022/06/12/flatpak-and-web-browsers/
https://librewolf.net/installation/linux/#security
https://github.com/uazo/cromite/issues/1053#issuecomment-2191794660
flatpak itself depends on namespaces, so saying that it works on any kernel is quite a stretch.
Can flatpak do this? This is a GIMP3 appimage running on ubuntu 10.04 without any container:
The kernel is so old that even the appimage runtime itself complains of missing functions and has to fallback to a workaround.
UPDATE: flatpak can’t work because bubblewrap itself can’t:
PR_SET_NO_NEW_PRIVS
is only available since kernel 3.5