It's immutable in the sense that the system files are generated by Nix and they're read-only. So, if you want to make a change to your system, you change the Nix files and rebuild the entire system. The old version of the system remains /nix/store and you can rollback to it.

You could also call it "declarative infrastructure", because you declare in your configuration.nix how the system should look like: users, packages, file paths, etc.