You might have misunderstood "postInstall" -- rather understandably so since elsewhere (debian packages, ...) for precisely the issue discussed in the article: arbitrary scripts executed after package installation.

In Nix postInstall (and preInstall, as well as preBuild/postBuild, etc) specifying commands to execute before/after the corresponding "phase" -- so if a package is "almost" good to go with just "make install", you could use postInstall to do something like copy a file omitted by upstream's installation target.

The point is that postInstall in Nix is part of how the package itself is constructed-- in contrast to commands run after installing the package. There is no equivalent for this in Nix in a fundamental way (not by policy or for technical reasons).