Lamanites in the United States

"The Book of Mormon is a record of the forefathers of our western tribes of Indians; having been found through the ministration of an holy Angel, translated into our own language by the gift and power of God, after having been hid up in the earth for the last fourteen hundred years, containing the word of God which was delivered unto them.  By it, we learn, that our western tribes of Indians, are descendants from that Joseph that was sold in Egypt, and that the land of America is a promised land unto them, and unto it, all the tribes of Israel will come, with as many of the gentiles as shall comply with the requisitions of the new covenant."

Joseph Smith is not making a statement that the Western Tribes were Book of Mormon people, but are the descendants of Book of Mormon peoples.