That's not "a huge number of packages", that's just the 1500 from the minimal installation. nixpkgs has over 80000. And amusingly enough, Nix itself is one of the (two) non-reproducible packages from the minimal install.

That’s great! This is only testing about 2% of the packages in Nixpkgs, but I bet if you weighted by package use that number would look a lot more impressive. Good work, maintainers!

But I stand by: this is the product of a lot of hard work by dedicated people working towards perfect reproducibility and not (as I thought the original article implied) something that Nix gives you for free.