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 "the supported version" from PyPI, as they don't want to deal with support requests for issues that are fixed later. They are likely worried that packaged versions will become stale, and users will expect support for specific versions, which they are unprepared to offer.
I think the offers from NixOS of having to enable config flags to get this enabled which make it clear it's not supported by upstream should be fair. Part of open source is that others are free to modify your code, and if you have a problem with that you're not really ok with the idea of open source.