Even though the developer does half the work, the system of 'Nix-using developer + Nix' is still constrained by NP-completeness, so in practice the two combined

Even though the developer does half the work, the system of 'Nix-using developer + Nix' is still constrained by NP-completeness, so in practice the two combined will not be able to solve the 'dependency resolution problem' in its full generality without a potentially exponential search. So the real question is, which assumption does the package of 'Nix-using developer + Nix' give up?