Roy A. West writes in Church textbook that the BOM is not a history of all of ancient America.
Roy A. West, An Introduction to the Book of Mormon: A Religious-Literary Study (Salt Lake City: LDS Department of Education, 1940), 11
c. History of Pre-Columbian Peoples.—There is a tendency to use the Book of Mormon as a complete history of all pre-Columbian peoples. The book does not claim to be such an history, and we distort its spiritual message when we use it for such a purpose. The book does not give an history of all peoples who came to America before Columbus. There may have been other people who came here, by other routes and means, of which we have no written record. If historians wish to discuss information which the Book of Mormon does not contain but which is related to it, then we should grant them that freedom. We should avoid the claim that we are familiar with all the peoples who have lived on American soil when we discuss the Book of Mormon.