We are copying here a module that I used for 1 or 2 days and ditched it afterwards (so it’s since then no longer available on my flake, just if you browse the history or if you find other likes to historic files). Also nevertheless as qtgreet is not in nixpkgs, and the thread author is not using the flake but has copied the code, a default value would also not have helped, tho I agree with you, that for production ready modules it make sense to have proper default values.
Edit: assumingly this was the source for the Rev pick of my flake Package request: QtGreet · Issue #235232 · NixOS/nixpkgs · GitHub