Well to be fair I think it’s because they aren’t trying to be NixOS. You could leverage those arguments against any distro that’s trying out an immutable flavor. Which is mostly accomplished through btrfs features.
I agree that Nix/NixOS does a lot more and it’s a genuinely impressive and paradigm shifting project but it does break with traditional Linux layouts and thinking in a way that immutability doesn’t necessarily have to do.
You could also make the same argument with the systemd and non-systemd crowd.
Either way I look forward to the future of both immutability projects and NixOS. I feel like both areas still need a bit of work but they’re both really exciting fields.