It is one thing for Joseph Smith to know that cement would be used by those in the Book of Mormon, but quite another to know they would also be exceedingly expert in it.
Bruce E. Dale and Brian Dale explain:
Not only were the Maya able to build with cement/stucco, they were “exceedingly expert” in working it, as explicitly described in both the Book of Mormon and The Maya (“astonishing,” “elaborate,” “magnificent” are the words used by Coe). This is certainly specific and detailed. It also is clearly unusual. The dominant view of the white settlers regarding the Indians in the early 1800s was that they were savages. How did the author of the Book of Mormon correctly “guess” that these “savages” could work so expertly in cement?– Bruce E. Dale and Brian Dale – Joseph Smith: The World’s Greatest Guesser (A Bayesian Statistical Analysis of Positive and Negative Correspondences between the Book of Mormon and The Maya)
Why would Joseph Smith say this? How did he know so much about the ancestors of American Indians?