The key point is that the Nix scripting environment (think “a Ports manifest”) is an intentionally restrictive language intended to have deterministic results for every operation. It’s not intended to be a Turing-complete programming language; that’s the whole point.

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.