B. H. Roberts allows for the possibility of other peoples in ancient America; notes that others in his day promoted a limited geography.
B. H. Roberts, Letter to William E. Riter, February 6, 1922, in B. H. Roberts, Studies of the Book of Mormon, ed. Brigham D. Madsen (Salt Lake City: Signature Books, 1992), 53–54
Moreover, there is also the possibility that other peoples may have inhabited parts of the great continents of America, contemporaneously with the peoples spoken of by the Book of Mormon, though candor compels me to say that nothing to that effect appears in the Book of Mormon. A number of our Book of Mormon students, however, are inclined to believe that the Book of Mormon peoples were restricted to much narrower limits in their habitat on the American continents, than have generally been allowed; and that they were not in South America at all.
If this be true, it might allow of other great stretches of the continents to be inhabited by other peoples, with other cultures and languages, which would still further tend to solve the difficulties of the Book of Mormon in regard to the existence of the great diversity of language stocks among the American race.