Where’s that XKCD comic about competing standards again?
I believe that this developer is entirely comparing apples and oranges. A config management system is a system that supports every single distro that is and potentially can ever be in your care. That means different inits, package managers, and even application versioning.
Nix is a package manager that’s ironically named because not only does it do package management but it has a tacked on level of config management. While it’s a neat idea that isn’t seen much in mainstream right now, it’s an entirely different beast than one that tries to unify distros, not in essence replace them.