it isn't. frankie confirmed this specific screen was taken directly from in-game gameplay segment
as chief is getting out of his cryo tube and walking over by cortana, ready to explore.
That means the screen shot was taken from a cinema, which always looks better than actual gameplay. Even if it is using the in game engine, its still a cinema, which again, always looks better than actual gameplay.
Actually, more than 2 years. The Reach engine was developed long before Reach's release (obviously) and 343 has been working on Halo 4 since Bungie decided not to renew their contract in 2007. Obviously, there's a lot more manpower now than there was back
then, but who knows. They could have been working on modifying the engine from the second they first got to start working on the old one.
The question is, are we doing the walking or is the game? Recent Halo games have a way of turning off a few of the pritty graphic effects as soon as we take control. If we are actually controlling it then I'm impressed with how far they have taken the Reach
engine in two years.