While that is true, it’s also r13y on another level: Reproducible evaluation. That mostly stems from pure eval and locking.

In the “before times”, you’d get your Nix expressions from some mutable location in the Nix path, so running i.e. a nixos-rebuild on your configuration could produce two different eval results when ran at two different times, depending on whether anything about your channel configuration changed in the mean time. This cannot happen with flakes as all inputs are explicitly given and locked.

You could achieve the same using niv etc. before but that had its own issues.