That's the part I always feel when looking at FP languages. They sound good on paper, examples are very tempting, but when reality kicks in to this pure, predictable, perfect world, it turns into a massive pain.
What the author of the script has done here, you’re supposed to do by writing code in some other language that generates a Nix manifest (or just by hand-rolling a Nix manifest.) And yet, the author here managed to get Nix to non-deterministically generate Nix.
Considered from the point of view of Nix’s goals, this is more an “exploit” than a truly-needed feature.
Generally speaking these sandbox determinism requirements in Nixpkgs/NixOS are not annoying, they are a crucial feature: you know what you get when you install something. But yeah, it is a constraint, and sometimes when you try to package some weird program where the Makefile does some arbitrary network operations, you might find it annoying -- and you can locally disable the sandbox -- but the whole Nix philosophy is that build scripts should be reproducible, so then you just have to fix it.