Usually when steam refuses to launch, it's because there's some Steam process that's borked but still running. Most of the time, a simple pkill steam fixes it (yes, that includes for flstpak`).

As mentioned down thread, the runtime isn't your problem. The runtime is what's needed for native Linux games and I think is also used by proton (not used by Steam itself), so it's kind of like proton for native games. Steam doesn't use the runtime at all to launch.

If killing Steam doesn't work, try rebooting. If that doesn't work, try updating the flatpak. If that doesn't work, I suppose reinstall Steam.