12 Now behold, all those which were in the possession of Helaman were written and sent forth among the children of men throughout all the land, save it were those parts which had been commanded by Alma should forth.
This of course requires the people to have had at least one writing system.
The caveat is that some of the engravings had been commanded not to be given to the people. Mormon goes out of his way to mention that in publishing the words, Helaman did not disobey the commands of his grandfather Alma-2, which we can read in Alma 37:
27 And now, my son, I command you that ye retain all their oaths, and their covenants, and their agreements in their secret abominations; yea, and all their signs and their wonders ye shall keep from this people, that they know them not, lest peradventure they should fall into darkness also and be destroyed.
Mormon brings up these "secret oaths" on several other occasions, too. (See Helaman 6:25 and Helaman 8:3.)