Moroni has made promises to Joseph Smith and is doing so on Yahweh’s behalf. His authority is therefore a legitimate question. While Moroni cannot prove his authority, he does declare it.
Text: Ether’s second chapter in the 1830 edition ends here, with the conclusion of Moroni’s address to Joseph Smith. I deduce, as already mentioned, that Moroni had a vision of Joseph Smith as the future translator and created the written dialogue with him that appears in his abridgment of the book of Ether. The reason for that dialogue is clear, but not its placement. Why didn’t Moroni write a separate book of instructions or include the specific information for Joseph in his own text where he again returns to instructions about the future?
Moroni provides no answer; but I speculate that he saw the Jaredite story as a historical model for the downfall of the Nephite people: God leads a people to a chosen lands and protects them as long as they are righteous; but they become unrighteous and are destroyed, leaving behind them a sacred record engraved on plates. Because he saw history as cyclical and because the cycle had just repeated itself, Moroni saw himself as a connection between the Jaredite model, his participation in the Nephite repetition, and future inhabitants who must be warned about this cycle. To Moroni, the book of Ether was simultaneously ancient history and prophecy. It spoke to Moroni as past, present, and future. That perception positioned him at the pivotal point between past and future, moving freely between the two as he abridged the book of Ether. As history presented information for the future, Moroni spoke to that future.