It's just marked as unavailable. Apple does that to try keeping people from using XPC on iOS. Use the full code from GitHub, it has a bypass for that Xcode check
As far as I know, how the review happens is that reviewers just install apps onto their iPads, tap through all the screens they can find and make their decisions based purely on that. So if an app connects to server and asks what it should do, it's possible to make an app behave differently for reviewers and all other users.