We didn't like the standard Game Center pop-in notifications, so we designed our own - these use normal UIViews, and we use the built-in Twitter/Facebook sharing view controllers.
The loading spinners, alerts / dialogs are all in SpriteKit. I'm not sure if this is optimal (I believe other developers lean on UIKit a bit more), but it all fits together nicely this way.
One of the youtube movies on your linked page is titled "Hexiled Gameplay iOS & Android iPhone & iPad HD" and uploaded by "AndroidGameTech".
But writing my own game apps, I do realize why you're not doing that when implementing games directly in things like SpriteKit.
Are they using flash ? Unity ?
The music was something we used a freelancer for, however.
Hopefully if Hexiled continues to prove successful, we're in a position to get help on the visual side!
Being the first game I've ever made I wanted something I could jump into really quickly just to play around. Got a bit of momentum on it and kept going.
I do wonder about the future of SpriteKit because of this, since having to code it two or more times is a huge effort. Apple seem to be all-in on it though, especially with SceneKit coming soon.