Haven't developers and publishers, or people in general that want to advertise "something" been claiming this for a very, very long time?
IE; "ingame realtime videos" of a game rendered by the very console it's meant for; new consoles showing off videos and tech that look incredible -- but never really come close to the quality of the ad when it's actually out.
I think there's a massive, extreme line between "videos rendered by whatever, for whatever, on whatever in realtime", as opposed to actual gameplay videos, with someone playing the game and doing some sort of gamey actions; jumping, climbing, equipping items, picking stuff up and equipping it, fighting, evading, dodging someone or something -- and NOT just a camera floating through a world.
Lots of tricks, "sleight of hand" and such make up for the extreme gap between real world implementation and eye candy, pure fantasy commercials.
It might also be cause of, or have something to do with, the extreme amount of resources put into showing off a relatively short video/scenes, or gameplay that's meant to attract customers. Ie things that would just not be financially or logistically feasible to develop for an entire game by publisher or developer.