It's strange that they found this staggering when you consider how much contact there was between the northern Europe and the Ottomans. It's well documented that Northern European mercenaries traveled down through the Balkans and served in Ottoman armies. Some of them converted and rose to high ranks. I wish I could remember the name of the Ottoman Admiral that was from northern Europe but his name escapes me right now.
I know there was a significant amount of trade along these routs as well.