Cool idea. But I'm wondering if this is allowed in the App Store?
It was acquired by PushIO which was acquired by Oracle. Along the way the SDK stopped getting updates with new iOS versions and Apps stopped working.
The yearly updates of the iOS SDK will impose a bunch of work on SDKs like this to keep up. A good model for open source.
All the best to the author, might be a good model for a consulting gig around the open source SDK for sure.
The way this works is like how a web browser works, all the code you will ever need is already in the binary, the JSON markup is just used to custom-construct your own app to your liking in realtime.
Long answer: it depends on what you're using it for. You're allowed to use things like this, or React Native or Cordova/Phonegap and deliver updates OTA (without going through the App Store review process) as long as you don't "significantly change the app from what we see during the review process".
Of course, how to interpret this is up to you, and is always a danger on the App Store. But support of this model is promoted by Apple themselves by TVMLKit, an API for building native Apple TV apps via JS+XML, delivered OTA over HTTP.