Immutable ≠ atomic, but they generally come as a package deal. Bazzite, Silverblue, and all those other distro's that call themselves atomic are also immutable. An atomic distro is just one with atomic updates, and an immutable distro is any distro with a read-only core.
These distro's have started mainly calling themselves atomic because they agree that immutable is a poor description that generally confuses users.