I find that many Linux users have a misconception about immutable distributions without knowing what it actually is. There is a lot of misinformation and generalization in the Internet about immutable distributions being “locked down”, “inflexible”, etc., when we could argue the same with many traditional distributions. In this article, we’ll look at what makes an immutable distribution, the concept of an immutable distribution versus implementations, misconceptions about immutable distributions (both pro and con), and why they exist in the first place.
You're actually right that it seems to use parts of it:
As such, the packages look largely different:
(define-public hello (package (name "hello") (version "2.10") (source (origin (method url-fetch) (uri (string-append "mirror://gnu/hello/hello-" version ".tar.gz")) (sha256 (base32 "0ssi1wpaf7plaswqqjwigppsg5fyh99vdlb9kzl7c9lng89ndq1i")))) (build-system gnu-build-system) (synopsis "Hello, GNU world: An example GNU package") (description "GNU Hello prints the message \"Hello, world!\" and then exits. It serves as an example of standard GNU coding practices. As such, it supports command-line arguments, multiple languages, and so on.") (home-page "https://www.gnu.org/software/hello/") (license gpl3+)))
vs
{ callPackage , lib , stdenv , fetchurl , nixos , testers , hello }: stdenv.mkDerivation (finalAttrs: { pname = "hello"; version = "2.12.1"; src = fetchurl { url = "mirror://gnu/hello/hello-${finalAttrs.version}.tar.gz"; sha256 = "sha256-jZkUKv2SV28wsM18tCqNxoCZmLxdYH2Idh9RLibH2yA="; }; [...] meta = with lib; { description = "A program that produces a familiar, friendly greeting"; longDescription = '' GNU Hello is a program that prints "Hello, world!" when you run it. It is fully customizable. ''; homepage = "https://www.gnu.org/software/hello/manual/"; changelog = "https://git.savannah.gnu.org/cgit/hello.git/plain/NEWS?h=v${finalAttrs.version}"; license = licenses.gpl3Plus; maintainers = [ maintainers.eelco ]; mainProgram = "hello"; platforms = platforms.all; }; })
Also note that NixOS' repository is one of the biggest among all distributions while Guix System only hosts free software.