Took me a moment to digest that flake.nix, but it just builds the nixpkgs python package with a bunch of overrides for each version of python applied.
In other words, @kzvi, yes - unless I’m very mistaken.
@domenkozar in fact modularized the upstream python definitions to fully accept building various versions, including the passthrus, which include the withPackages function: https://github.com/domenkozar/nixpkgs/blob/b19f607a1b1d36a02741cf3aefe674bd3848fcf6/pkgs/development/interpreters/python/passthrufun.nix#L86
This is very cool 
I don’t think it caches pip packages, though, so you’ll need to “build” those downstream?