The book headers in Nephi, and later in Mormon’s edited chapters, suggest that they were written prior to the content of the book. They serve as an outline of the intended content. They do not necessarily represent all of the content. For example, the book header for 2 Nephi spoke only of the historical events. The book header has nothing that covers the material from 2 Nephi 6 through 33.
This makes the header for the book of Jacob more interesting. We will see that there is a gap between the content of Jacob’s sermon and his account of the incident with Sherem, yet both of those are included in the book header. This suggests that both were intended to be included, and therefore suggests that the whole of the book of Jacob may have been written after the header, and therefore after the incident with Sherem. That makes the entire book of Jacob a reminiscence, rather than having been written during the events recounted.