The former priests of Noah were educated men, and they certainly could read. In antiquity, reading and writing were separate skills, but they clearly wrote. Mormon has them teaching the Nephite language and the ability to read and write to the Lamanites. While it is true that not all peoples in Mesoamerica had a written language, it would be surprising if the Lamanites in the region, suspected to be the land of Nephi, would not have already understood how to read and write. Mormon is indulging in a little ethnocentrism and having the Nephites become the cause of the better aspects of Lamanite culture. While likely not historical, it is certainly a technique that fits comfortably into ancient historical writing.
We also see that the Lamanites continue to fit the basic Nephite stereotyping of the Lamanites. They “began to be a cunning and a wise people, as to the wisdom of the world, yea, a very cunning people, delighting in all manner of wickedness and plunder.”