I stepped through the video and can confirm that Jason gets more information on his screen than I do on my iPad Air 2. So yes, Scrivener for iOS does take advantage of the 12.9″ screen.
The word has been given that Scrivener for iOS has been sent to Apple for review. Sadly, there’s no telling how long Apple will take to do their Approval Dance(tm).
BTW, the price in the video is wrong. Scrivener for iOS will sell for $19.99 USD in the App Store.