Due to the complexity and breadth of the iOS APIs, I have my doubts that this is a reimplementation, especially one completed by one developer.
If that's not the case, this would be technically feasible, but of dubious legality.
If that's not the case, this would be technically feasible, but of dubious legality.
Most apps, for obvious reasons, use only a tiny fraction of the platform. And those bits they do use aren't always critical. Facebook, for example, wants location information but would be happy if you just lied to it and gave it a hard coded value.
But yes, quite an amazing achievement for just one person.