I wouldn't say that this demo proves that Uncharted has moved towards being an Open World game. Yes, you see your destination but it appears you are still being sort of forced in a general direction like the other games. Furthermore, Naughty Dog would have been stupid if they spent any time trying to show off how lifelike their invisible walls are in the game. Also, I doubt the whole game is going to end up taking place in just this one environment though I could be completely wrong on these assumptions.
Frankly, having played the last 3 on PS3, I'm not even beginning to draw the connection between the view of your destination, and an open world title. To be even more frank, I'd state emphatically that anyone making this claim, has never played one of the Uncharted titles, because that moment where Drake looks out over a broad expanse, and the destination is somewhere off in the distance, is literally in _every_ Uncharted game.