The Last of Us VGA trailer was In Game, proof inside.

[Hephaestos]

why do you even need proof?
This is a cinematic, so the amount to be loaded at any one point can be precalculated and optimized. If other rooms were loaded permitting the characters to move, this would require more resources. If all the possible character animations were loaded instead of a scripted sequence, this would require more resources.

In no way is this cinematic an example of what the game will look like with actual gameplay... though it does point in the general direction.