That article on "What is Nix and .. " does not contain the words "Nix is a..." A quick read gave a vague sense that it was about installs and file paths so maybe something package-related? Nothing definite.
If you click through to https://nixos.org/nixos/nix-pills/why-you-should-give-it-a-t...
you will see "Nix is a purely functional package manager and deployment system for POSIX"