Yeah i'd laugh too. But we did the same thing with GTA V until we finally got some map details just before launch, and that turned out pretty accurate.
I still think we would have been pretty accurate with our own map if the leaked one didnt exist. It's just aided us along the way this time. I wont lie, the minimap in the gameplay trailer has been a massive help in determining the placement of towns and landmarks on my map. Even if we do have a leak to match it up with.