SteamOS is immutable with the option to disable the read-only system. Bazzite is atomic because with the use of scripts you can modify the system in ways that do not break stability. You can view the FAQ if you’d like: "Since Bazzite is a custom Fedora Atomic Desktop image, it makes use of read-only root files for stability purposes, and is built with libostree which has advantages such as:

• Low risk of an unbootable system
• Rollback system updates if necessary, and the ability to pin your current deployment as a backup save state without losing user data.
• Smooth upgrade process from major Fedora point releases.
• Layer Fedora packages to the host that survive between updates.
• Focus on containerized applications that do not interfere with your host system." I’m not associated with them I’m passionate about the usability & convenience of their implementations.