It's state at a different level than most configuration management software deals with. When Puppet reasons about the state of an existing system, it asks thing

It's state at a different level than most configuration management software deals with. When Puppet reasons about the state of an existing system, it asks things like 'what's on the PATH right now?'. NixOS doesn't— it reconstructs the directory tree under /run/current-system/sw/bin without asking that. Same with 'modification' of files that live under /etc. What you and the GP write here is nuance worth admitting, but I wanted to highlight what people are getting at when they say Nix's approach is 'stateless'. What they mean is that Nix's approach to state is really qualitatively different, and hands state off to be managed by a lower level part of the system as much as possible.