Nephi brings closure to this experience in the only appropriate way possible. He testifies to the truthfulness of what is written.
Reynolds and Sjodahl suggest that the closing "and thus it is. Amen" might suggest that this was a sermon: "The closing words may indicate that the foregoing, from the beginning of chapter 11, was intended as a sermon and was delivered as such. Undoubtedly, the family of Lehi held regular services on the Sabbath, and the vision, as related, would naturally form a fruitful subject for discourses."
While this might be a reasonable speculation, there is really nothing in the text which so suggests. The nature of the apocalyptic vision, and the references therein appear more directed to a modern audience than an ancient one. There are no other indications in Nephi's writings of his own teachings or sermons. Nephi will include his brother Jacob's writings and discourses, but never indicates any of his own. The particular mentions made of the restriction of themes are so related to the latter days, that it is difficult to believe that this particular vision was (in the form presented) given as a sermon at the time. The vision reads much more as a personal experience for Nephi, and one which is recorded for his personal reasons in a set of plates which are separate from the ones he had obviously already worked on. The suggestion that this might have been a sermon does not fit with the presentation of the text.