It's not really clear that the scene is meant to be a battle, and, as it's a pre-rendered scene, it's entirely symbolic, anyhow. Kairi and Namine should at least be there for the sake of having the entire main cast of protagonists in one place.
Again, it's not necessarily a battle against Xehanort, it's just part of the opening sequence. While it probably foreshadows something that will happen in the game, it's not meant to be taken literally, so any of the characters could be there, even if it differs somewhat from what will actually occur in the story. As such, I demand Axel, Kairi, Namine and Xion in HQ CG in the same place with every other hero in the story NAO.
Additionally, it's possible all of these characters are actually there and we just didn't see them in this relatively short clip from the trailer.