We have contingency plans to fight wars against everybody in every conceivable circumstance. Some are pretty far flung. How could Hitler drop paratroopers in the Canadian Rockies when he didn't have one plane that could fly across the Atlantic? Let alone across the continent to boot.
England, Iceland, Canada. Then the U.S.. If Germany had won the war in Europe and taken England, it would have had the shipbuilding resources to build a monstrous fleet if it chose.
Even assuming that the author is right, and that an invasion may not have ultimately been successful, can you imagine the cost in lives of defeating the Wehrmacht on U.S. soil?