I don't think that would actually make sense. I'm sure the backend/business logic/functionality of settings has extremely little in common with iOS, so that can

I don't think that would actually make sense. I'm sure the backend/business logic/functionality of settings has extremely little in common with iOS, so that cannot be reused with catalyst. And the UI is tweaked and "improved" for macOS (compared to iPad UI). So you don't actually gain anything from using the iPad settings app on macOS through catalyst.