When Nephi wrote the book of 1 Nephi, he clearly had an outline for the entire book. There was a long story he wanted to tell, and he wanted to tell it in a particular way. That plan also drove the early chapters of 2 Nephi. In contrast, Jacob has no plan for his book. It exists in three discreet sections. This first is a sermon. The next is a different sermon on a different topic, and then finally there is his conflict with Sherem. There is no cohesion.
This closing statement would have been a fine ending for the book of Jacob. The declaration that these are the plates of Jacob, and that he couldn’t write everything, are both themes for conclusions, not beginnings.
It appears that in contrast to Nephi’s plan, Jacob wrote in sections, probably with time between writing. Thus, he writes three incidents and had no plan for future text. It is possible that the next sermon had not yet been given when Jacob wrote this part of his record, contrasting with Nephi’s retroactive writing of his history, which didn’t catch up to real time until 2 Nephi 5.