Immutable doesn’t mean no updates. It just means that the user can’t change the system, and also doesn’t have to. An immutable System can have updates every night, as long as they are compatible.

It’s a bit like iOS and android, the system is the same on all devices of the same model, there are just different configurations and different apps.

That doesn't sound like "immutable" to me, more like batched updates. I guess like Windows service packs?

It's as immutable as burning a new bootable LiveCD every time you update except that the images are kept on the HD for convenience and speed. Once booted, files stay pristine until the next reboot.