This is mostly a matter of maturity and whether there is tooling to make this possible. As such, it's not surprising that only the more established atomic distros are part of the conversation. Those^[Factually speaking, Guix System is even older than Fedora Atomic. Hence, it deserves to be mentioned alongside the other two. However, I am simply unaware of a gaming distro that's based on Guix System. Which is exactly why it wasn't discussed alongside the others.] being Fedora Atomic and NixOS.

Besides Bazzite, Playtron GameOS is also based on Fedora Atomic.

As for NixOS-based gaming distros, there's Jovian NixOS and GLF OS.

FWIW, ChimeraOS is IIRC a notable exception to the above.

Note that none of these come close to Bazzite community-wise*.