Nix doesn't "solve" reproducibility. For example, if the thing being packaged builds non-deterministically (e.g. if components are built concurrently, and added

Nix doesn't "solve" reproducibility. For example, if the thing being packaged builds non-deterministically (e.g. if components are built concurrently, and added to a tarball in whichever order they finish). Nix can make it easier to avoid introducing new problems, and it also gives an audit trail of which versions of things were used to build things. Also Debian doesn't use Nix, so that wouldn't solve the problem for Debian. Nix (along with everyone else) does benefit from the improvements that Debian is making, e.g. finding non-determinism in programs and getting it fixed upstream.