Why do you think there is no introductory dialogue for chapter 15?
The introductory dialogue in each chapter serves as a change in perspective and an added behind-the-scenes insight into Ender's training. Though the story itself is not a first person narrative, it is a limited perspective that follows Ender and keeps the thoughts and actions outside of his immediate knowledge a mystery. The introductory dialogue is just a small window outside of Ender's little world.
Chapter 15 does not include this dialogue, however, because the story is over. Ender's training is over. The war itself is over. When he wins his final battle and defeats the Buggers, all the mystery is revealed, for Ender and for the reader. Introductory dialogue is no longer necessary and it serves to show that the training, manipulation, and secrecy is over for everyone.