Yes, you're absolutely right, but you're also illustrating the author's broader point a little bit. For someone not very deeply familiar with Nix (the language), that is a very difficult one-liner to parse. You have to be pretty hard-core nix-pilled to understand what that one is doing just reading it straight-on (my NAS is using NixOS, so I've written some amount of Nix). On the other hand, the TypeScript code the author provides is reasonably straight-forward, you don't have to be particularly familiar with either TypeScript or Nix to understand what it is doing.

You're correct in that the article is wrong in saying you can't use a nix repl this way, but I took the broader point of it to be "Nix-the-language gets in the way of the Nix-the-model". Which is something that I personally agree with 100% and I think the article illustrates beautifully.