Well, they are and they aren't.
0:48 and 0:53 show what looks to be like the opening scene for the C1 level "Relic". (The one where the Alien grabs Prophet) However, there's some stuff wrong with this footage in the trailer.
First off, I will show you some genuine Crysis 1 screenshots of that very cutscene in Crysis 1 (Taken by me via Steam's new screenshot feature! )
This is what the Normal Scene Background looks like:
Now here it is with Prophet, with his suit that begins to malfunction:
Now here is this exact pic but in the Prophet Returns Trailer. (at 0:48) Notice the frozen ground, and the lack of trees.
This is at 0:53 in the trailer, when the Alien drags him away:
And this is where the Alien Drags him away in Crysis 1. It's hard to see, but the Alien Drags him off to the left instead of straight forward.
As you can see, there are some pretty major differences. My guess is that they have imported all of the old Cryengine 2 Data of Crysis 1 and put it into the Sandbox 3 Editor and done a little re-mapping. Not 100% solid evidence that you can import things from CE2 into CE3, but it sure points in that direction.