Looking at the ‘git blame’ of that flake.nix, the systems was written in mid-2021. My guess is the omission of aarch64-darwin was just its lack of popularity at the time.
I was able to add aarch64-darwin as a github runner to a Gradle-based project of mine, and it built just fine.
However, generally, my understanding is that Apple is less popular on Nix, and whatever works is thanks to some heroic efforts by volunteers. c.f. Nix 🖤 macOS Monthly