That doesn't make sense. A reproducible build means that a build done in two different places results in the same binary. It has nothing to do with whether the vendor distributes binaries (with optional source) or source alone. The binary is what matters. i.g., if I build this package myself, will it result in the exact same binary that I download from the vendor?

Binaries can be signed to prove that the chain of trust is unbroken between the vendor (who maybe you trust enough to run their software on your systems) and you. In the case of a system with reproducible builds, that signature also says, "if you were to build this yourself, you'd end up with the same binary I am sending you."

Edit: I am beginning to realize that when davexunit says "pre-compiled binaries" they are not talking about an OS vendor (like Guix) distributing a binary package (with source available). They are talking about binary blobs provided by third parties, and in a form that is not reproducible, such as an nvidia driver kernel blob. This has led to me assuming davexunit is nuts, but apparently it is just that neither of us has been speaking very clearly to each other, and thus we've seemingly been talking past each other.