The term "earth" as is used in Helaman 14:27 referred only to the land in which the Book of Mormon peoples lived. It wasn't a reference to the "world". I think this is even more proof that the Book of Mormon is true and that Joseph Smith didn't write it himself. He knew the bible very well and would have immediately recognized the inconsistency if he were writing it himself. Because he was translating it from ancient authors, he didn't give it a second thought.