I doubt that particular behavior has changed in the past. It’s a fairly static thing across distros, and the shadow file is kept deliberately imperative for security reasons.
I think the principal thing to take away is that it’s a fairly advanced thing to do, and the guide isn’t really a guide as much as it is a blog post showing a cool pattern. While being explicit about it would of course be better, various ways of handling passwords declaratively exist and the post probably just assumes you’re experienced with nix, NixOS and linux systems in general and are using any of them.
Not great for someone who’s figuring it out of course, and you’re right an addendum would be nice
While we’re at it, GitHub - nix-community/impermanence: Modules to help you handle persistent state on systems with ephemeral root storage [maintainer=@talyz] is worth a look for anyone wanting to do this, they also have a “further reading” section in the readme, which contains both @grahamc’ s post and a slightly more tutorial-ey one (that does explicitly tell you to use declarative password management - though it’s probably not actually a good idea because those passwords are always world-readable if they are declared that way).