This has a big "FAKE" sign written all over it, like the infamous Killzone 2 PS3 "demo" -
http://www.youtube.com/watch?v=dPkgyr46wpM
For starters, this video is 640×360. Not only this doesn't match the aspect of 3DS's top screen, but also has higher vertical resolution. Second, if 3DS was able to generate graphics of this quality, why would MGS and Kid Icarus demos look much worse ?
My guess is that what they shown at E3 was a full motion video, prerendered from different angles (3D panorama). You can still look around, which gives the false impression of the scene being generated in realtime . Check the link here:
http://www.labnol.org/internet/video/3d-vi...the-video/3043/
PS. "The trailer is rendered in real time using the game engine" - that might be true, but NOT ON THE ACTUAL 3DS HARDWARE.