The video is not fake but.....
the video is about 8 years old. It is part of the japanese movie teaser for "Myuutsuu no Gyakushuu/Mewtwo Strikes Back". I doubt that the animators had any plans for Pokemon DP at that time. ;)
For those interested the full video can be found here:
https://smg.photobucket.com/albums/v466/unown_/attach/?action=view¤t=g0_EwX-OzlQ.flv
Though I'm not sure if it is the same person, the looks are the same though.
In a lot of Pokemon movies, there are scenes that never made it into the actual movie. Those scenes are concept animations for the animators to test out a few thing out or so. Trailers usually contains a few of those scenes. A small list:
movie 2: Ash jumping in pursuit of Pikachu from a cliff, while Misty burst out in tears and Tracey trying to stop her.
Movie 7: Ash and Max being hit by Deoxys attack at the top of the battle tower
Movie 7: Ash kneels down with an injured Pikachu while Deoxys emerge from the smoke.
Movie 10: Ash and Pidgeot flying over a bridge
My guess is that the scene from the trailer is one of the concept scenes.
EDIT: a bit too late. I'd really need to type faster.