Not sure what you mean by that, the Nix packages that are reproducible have reproducible binaries.
In the Nixos world there isn't really a concept of a "binary/installation package" like in Debian or elsewhere. Everything can be rebuilt from source on any machine, but because everything is hashed, if the official binary caches have already built something with the same inputs, they can just give you the outputs directly. So it's more like memoization than a .deb or something that you install.
Nix is a functional language that builds recipes (derivations) to build stuff, with all the inputs and outputs hashed. If the derivation you want to build has already been built by a cache you trust, the system will just fetch it instead of building locally.
What the Nix reproducability project checks is that the same derivation produces the same output regardless of what machine it's built on.
> In the Nixos world there isn't really a concept of a "binary/installation package" like in Debian or elsewhere. Everything can be rebuilt from source on any machine
Thats not actually the case. A derivation is just an abstract concept that combines a hash with a "thing". Here is an example [1] of a pre-compiled mono binary that gets downloaded and installed (after patching paths).
Debian has been building in a clean sandbox with only required, tracked dependencies since decades.
It's also building the large majority of packages reproducibly including the binary and whole installation packages (not just the sources like nixos)