Mormon appears to have mixed his sources in chapters 1–7 in 3 Nephi. At this point, however, he declares that he will be concentrating on a single source that was not part of the large plates. Although he does not identify Nephi as the writer, it is clear from the description that “he truly did many miracles in the name of Jesus.” Mormon had recently written about the miracles Nephi performed in 3 Nephi 7:19.
Because Mormon is using a record that didn’t come from the large plates, he wants to testify to his readers that the record is still a true record. This is almost certainly the “shorter but true account” that Mormon mentioned in 3 Nephi 5:9, where Mormon did clearly name Nephi as the writer of the account.
It is perhaps because this record is separate from the annalistic record of the large plates that Mormon must also testify to the accuracy of the dating. It is a record that is not tied to the other years, but Mormon declares that there is no reason to believe that the dating is incorrect.