See, those screencaps strengthen my theory.
Guerrilla had the alpha kits since november. Then they got this base engine
code up and running. So, basically only the graphics, no AI or gameplay mechanics.
Getting up to that point probably took them somewhere around April or begin May. So, they knew they had to show something at E3.
They took that alpha code engine, animated the whole sequence and recorded it at visual settings (amount of particle fx, quality of textures/shaders etc) according to what they see as feasible on final PS3 specs.
So, yes it is most likely a movie clip recorded from the engine and that\'s why you see those graphics errors in it which would NOT happen in a prerendered CGi movie as we know it.