Alma’s proof that he does not “know these things of myself, but it is the Spirit of God which is in me which maketh these things known unto me” is to recount his conversion experience. Of course this is precisely the introduction and proof we saw in the charge to Helaman. The difference is that Helaman receives a highly literate, chiastically structured accounting of that event, and Shiblon has only a terse reference to it. Why is there such a difference?
Ultimately, we do not know. We can only speculate that the calling of Helaman as a future prophet/keeper of the records would have necessitated a deeper understanding of the events and the process, where Shiblon needed only the reminder. This was a story that had to have been familiar to both of them, and for Shiblon the reminder sufficed. Alma deemed it important to more completely engulf Helaman in the experience, perhaps as a precursor to his future responsibilities.