It seems like a close comparison to me. The inputs to nix derivations aren't always directly named after the build flags they control, but ultimately they cont

It seems like a close comparison to me. The inputs to nix derivations aren't always directly named after the build flags they control, but ultimately they control build flags. If you disable the binary cache then you're building everything yourself with system wide control of build flags. Purity has to do with whether you let those builds depend on things that found lying around on the system versus things that are explicitly in the derivation (i.e. a pure build can tolerate missing files by building them, an impure one may get stuck for lack of a dependency or because a dependency was not as expected).