Nephi has enough support among those gathered that he can continue. He invokes Moses as an example of God giving power to one man. It is important to the context that Nephi specifically uses the opening of a passage through the sea as his example. Out of all that Moses did, why that event? Why not Moses receiving the commandments on Sinai, if Nephi were to speak of his prophetic calling?
The reason is thinly disguised, if at all. Moses led his people away from a government that, while rich, did not follow God’s commandments. Nephi is declaring that it is his purpose to similarly lead his people away from the rich, but religiously incorrect, Gadiantons who governed them.
In verse 12, he brings the story back to the firm Nephite base: the promise of the land. He may be declared a prophet, but he is only affirming the covenant that God made with Lehi.