Redaction: The first part of the record of Ether contains some account of events from the time of Adam to the time where the story begins in earnest. Moroni’s statement corroborates the only other information we have about the content of the plates, given as part of the description of Mosiah’s translation of the record:
Mosiah 28:17
17 Now after Mosiah had finished translating these records, behold, it gave an account of the people who were destroyed, from the time that they were destroyed back to the building of the great tower, at the time the Lord confounded the language of the people and they were scattered abroad upon the face of all the earth, yea, and even from that time back until the creation of Adam.
It is interesting that this is also a part of the record on the Coriantumr stela:
Omni 1:22
22 It also spake a few words concerning his fathers. And his first parents came out from the tower, at the time the Lord confounded the language of the people; and the severity of the Lord fell upon them according to his judgments, which are just; and their bones lay scattered in the land northward.
The Coriantumr stela does not have the history returning to Adam, but it does corroborate the description of the “first parents” coming from the tower, something that is important in both accounts. Some tie to the ancestors or founders is a common function of Mesoamerican records. The record tied the current king/people to their illustrious progenitors to establish the legitimacy of rule. Thus it is not surprising that two different records on different material forms and written for different purposes, might both have a connection to the ancestral past. Indeed, it would be almost required of such a document in a Mesoamerican context.
Moroni indicates that the record of Ether shows such similarities to the record of the Nephites, or the brass plates, that he does not feel compelled to include the duplicated material. The parts that Moroni leaves out are parts that come from Mosiah’s translation, and Mosiah shared with Moroni the identical set of brass plates against which any similarity might be measured. Based on what we know of the way Joseph Smith translated the plates of Nephi, we may expect that a similar method was used by Mosiah. Thus when Mosiah saw a similarity of content, he used the specifics from the brass plates, just as Joseph Smith did for Isaiah and the 3 Nephi Sermon on the Mount material. Thus when Moroni sees the text, it really is nearly the same, because the text Moroni has and the one Mosiah would have used as a model would be the same. It would be a dangerous assumption to posit that Mosiah used a better or more accurate or literal translation method than did Joseph Smith under similar circumstances of translating a document from an unknown language through the medium of the same urim and thummim.