The game's director, Tetsuya Nomura, points out that part of the plot was revealed in the TGS Closed Theater trailer. He notes that the discussion seen between these two characters in that trailer focused on their abilities to "see light." Noctis seems afraid to talk about these abilities, while Stella is very frank and opens up the dialogue by asking Noctis about how he obtained his ability.
It's rather funny to think that we now know slightly more about Versus XIII than we do of the main XIII title. That's especially harrowing after knowing Versus XIII isn't expected until after XIII is out.