By way of review, the prophet Zenos was an Israelite prophet whose writings are not contained in the Old Testament. We only know of him through the Book of Mormon. His most famous contribution to the scriptures is the allegory of the olive tree (Jacob 5). The Book of Mormon also tells us that he taught plainly of the Savior, that mercy is bestowed through the Son (v. 13), that there would be three days of darkness in the Americas, as well as destructions, earthquakes, and tempests associated with His death, and that Israel would be persecuted and scattered prior to their great gathering (1 Nephi 19:10-16, 3 Nephi 10:15-6). Finally, he was martyred for his valiant testimony (Hel 8:19).
The tone of Alma's question indicates that the Nephites had access to the scriptures. Even the poorest of the Zoramites are asked whether they have read the scriptures. Whether these records were kept on parchment or metal is unknown but their scriptures were more available to them than we sometimes imagine. The vision of the tree of life is not universally applicable if the prophets are the only ones who have access to the iron rod.