Hm. I know there were Breen in that prison. But it was run by Jem'Hadar. Maybe we can retcon it to say he had a Breen cellmate and their relationship was … complicated. 
Or we can all just agree to pretend that line in Chapter 30 really says "prisoner of the Dominion" and let it go.

Or we can all just agree to pretend that line in Chapter 30 really says "prisoner of the Dominion" and let it go.