This is great. Having access to all apps is nice, but it is also useful to know how and if flatpak apps are verified.
This mostly means they are packaged by official developers. This guarantees better security, as the chain of trust is shorter, and better support.
Yeah I get that SteamOS wouldn't, but Valve themselves have explicitly stated people should use Flatpaks, not distro repos or Snaps (perhaps with an exception for Arch repos if what you say is correct).
Seems very weird to me.