NixOS has atomic transitions between independent configuration states. So for a simple example, when you uninstall a program from your declared NixOS config and request a switch of configurations, NixOS does not take into consideration your current configuration so that it can uninstall those removed packages. Instead, it generates a brand new filesystem tree from scratch which does not include those packages. Similarly, configuration files are never edited in place, even with a guarantee of idempotency; they're regenerated from scratch.
Deployments of server applications do usually involve some inherent state, like the contents of a database or the fact of which services are running. In that respect, some deployment tools in the Nix ecosystem are not isomorphic. But the software management and package management components, for example, are declarative without involving any reasoning about the current state. It is in that sense that those components are 'stateless' in a way that the rule of thumb proposed in the GP is not.
Deployments of server applications do usually involve some inherent state, like the contents of a database or the fact of which services are running. In that respect, some deployment tools in the Nix ecosystem are not isomorphic. But the software management and package management components, for example, are declarative without involving any reasoning about the current state. It is in that sense that those components are 'stateless' in a way that the rule of thumb proposed in the GP is not.