As a "common user", I'd say very much yes. It goes quite deep into the internals, and touches underlying ideas and basic mechanisms of Nix, which I don't think are expected to change [ever].

That said, note that it starts to be relevant when you're starting to write Nix expressions (meaning, write something in Nix language). Sometimes also when you want to read them and understand them. Which... is probably quite soon after starting to use Nix/NixOS, if you'll want to tweak some configs or add custom new packages. If you're really, really just doing first steps with Nix/NixOS, and/or just use the default packages with only simple tweaks, I'd say you don't need to read it yet (I personally wouldn't grasp enough basics then to understand WTF the articles are talking about at that point).

I would suggest it, there is not much that has changed about the core of nix/nixpkgs, the nix language is quite stable and most changes have been additions rather than changing existing features.

The Nix language hasn't really changed, so sure. Note that it doesn't say much about NixOS.