Atomic/Immutable - clarification

I've recently tried Fedora Atomic spin (Kionite), and I was under an impression that atomic and immutable are the same thing. But I got this feeling that maybe it's not.

Kionite is definitely atomic. All updates are installed on a non-active image, and the new image is loaded after a restart. However, it's most definitely mutable - I can install whatever RPMs from any souce, on the system level. It's just requires a restart to take effect.

I haven't used Bazzite, but from what I heard, it is immutable and it's not possible to install whatever random RPMs. You actually must use DistroBox and Flatpak.

Am I getting this right? Kionite is atomic. Bazzite is both atomic and immutable.

I'm guessing, the main advantage of an immutable distro is that it's even more difficult to break it. It's also probably more sandboxed and should be more secure. But would it really be a significant advantage for a regular user?

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