The book says: given that it’s very hard to gain leadership in a category where competition already exists, it’s better to create a product in new category than trying to attack existing categories.
Category doesn’t have to be radically different, e.g. if there’s dominant player in imported beer, one can become the first to import light beer. If one can’t be the first to fly over Atlantic, one can still be the first woman to fly over Atlantic.
See also Eric’s take