After the Nephites take ungodly oaths in chapter 3 of Mormon, Mormon says that he “did utterly refuse from this time forth to be a commander and a leader of this people” (verse 11). Yet, we find him in chapter 5 repenting of that oath and leading the Nephites into battle again. If this book was written at the same time, why wouldn’t Mormon say that he refused to lead the people “for several years” or something like that rather than making an absolute statement only to contradict it a bit later? The text of the book indicates that it was written after all these events were in the past (6.6), “therefore I made this record out of the plates of Nephi…”, which happens at the end of Mormon’s life.
The reversal of Mormon’s position instead is consistent with someone who is inventing the story as he goes along, not knowing the future events from the “present” events.