It's really not. It's the UE3 Mobile Engine. Which while it's compiled from the same C++ source, It has enough things stripped during compilation to not really be considered 'full UE3' custom shaders for instance are not supported.
And atop of missing some things from UE3, Citadel is designed specifically for limited devices.
While still a very impressive feat, let's not confuse this with running GoW or The Samaritan demo in the browser.
As mentioned in other comments, we also ran other UE3 games, like Sanctuary. We demoed that in a booth at GDC last month where people could play it. That's a full UE3 desktop game with bots, AI, normal FPS mouse control (not tablet-like), etc. etc. You can see it in action in the 2nd half of this video