alt Hacker NewsNix definitely does not allow most things to be built offline (at least in the way Debian means it).
With Nix, any fetcher will download the source. It does so in a way that guarantees the shasum of what is fetched is identical, and if you already have something in the nix store with that shasum, it won't have to fetch it.
However, with just a mirror of the debian source tree, you can build everything without hitting the internet. This is assuredly not true with just a mirror of nixpkgs.
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> With Nix, any fetcher will download the source.
OK, I see how the Debian idea differs from the Portage/Nix/etc. idea. For Portage and Nix it is enough that the build proper be offline, but the source code is fetched at the beginning of the package build. Not only do I find this sufficient, I prefer it because IMO it makes the package easier to work with (since you're only wrangling the packaging code, not upstream's).
There are probably still ways to maintain a source archive with a ports system. Just analyze the sources used by builds, create a mirror, and redirect fetches to use the mirror. It's not that crazy. The packaging would still be a separate affair.