It’s new to me, I think it’s saying that your system is built up by you declaring what you want in a file, a single source that everything comes from.
It’s atomic because each action the system takes is carefully completed rather than bailing out and requiring you to fix something.
It’s immutable meaning you declare how you want things to be set up and then critical changes stem from those declarations and nothing else. You would obviously generate preferences, save data, etc. but the files that make the system / packages work are carefully locked.
It’s like the concept of flatpaks + structured system defining + modern common sense OS operations?
I wonder how many sites will bother checking for Spanish pornpasses. Seems they’re just playing people and waiting for the inevitable, “Turns out the Internet isn’t respecting our kids, we need to ratchet up the control. We tried to give you a good deal though, right?”