It is a very good book and i'd really recommend it, I'm debating wether to get his other books about the Hortens. The dogfight thing came from a pair of people that I spoke with about the 229 and maybe a book I read as a kid- Basically it's a story i've heard a number of times from different places but got curious about confirming or proving it false.
I agree that the battle would have been quite unlikely, though I won't rule it out as totally impossible given the rather unorthodox nature of the whole Horten operation and chaos generally in Germany at that time.
One other thing from a review of the 229 books he wrote-
"Another refreshing voice is that of Rudolph Opitz, also a pilot, and one with experience in unconventional aircraft. "Any one of the old timers who flew Horten sailplanes ... knew that the Horten all-wing flying characeristics are horrible," he said. "Anyone who came from the outside and flew the Horten all-wing aircraft found that the aircraft flew, but that was about all. The All these aircraft required considerable changes, but the Horten brothers would never listen."