Debian packages are also built from source, but their model for building and distributing packages has a different ontology, it considers different kinds of resources than Nix does.
> If apt could compile all its packages from source with an option to do so and a cache, it would be as reproducible.
This is simply wrong. There are many source-based package managers besides Nix: Portage, Sorcery, Pkgsrc, Ravenports, MacPorts, FreeBSD Ports, etc. Some of them, like Homebrew for example, even support binary caching. None of them have the same kind of reproducibility guarantees as Nix offers, except for Guix.
The counterfactual about apt doesn't even really make sense, as using its own special source archive format, having binary repos, and performing install time dependency resolution based on version constraints are all pretty essential to what makes it apt. But even so, the mere fact that it installs to global, shared locations, e.g., under `/usr`, means that there are circumstances where you can't guarantee that a package will always load the same libraries it was built against, namely in the case of transitive dependencies which had different versions at build time.
