Earlier quoted context omitted.
It's impressive because it's most likely fake.
You're probably being downvoted for the time of your post but I wouldn't be surprised if it's actually true. E3 demos are notorious for being pre-rendered moonshots far removed from reality once the game hits the market. Some more egregious than others. That said, if the actual game is only half has gorgeous as this video, it's still crazy good looking.
If you look closely, some scenes are photo real. Other scenes, like the giraffes and a few frames before that, seem to be in the uncanny valley.
I suspect that this game may be this generations Crysis. It requires ungodly horsepower to render at full settings, and perhaps a more-than-robust internet connection to transfer all the imagery data.