Is there greater context here that helps explain what's going on? Reading the comments in the thread didn't clarify much for me as someone unfamiliar with NixOS, their apparently unique Python packaging scheme, and Home Assistant.
It sounds like Home Assistant by default pulls the (EDIT: Home assistant specified, not latest) version of the package from PyPI at runtime and loads it dynamically. The point of NixOS is reproducible builds, that is if you build a given nix environment you will always get the same code. Dynamically pulling from PyPI at runtime defeats that. The author doesn't want any way of downloading the code other than getting "…
Why would users report to his project when installing Home Assistant? That's not usually how it works and Home Assistant has a lot of other dependencies.