I think, it's based on an old flake-compat package or something. It's not inherently bad, but it displays what I dislike the most about Nix design, it's very opaque and magical until you go out of your way to understand it.

The globals are another example of this, I know I can do with something; [ other ] but I am never sure if other comes from something or not. And if it's a package parameter, the values also come seemingly out of nowhere.