Mormon wrote the text at the end of Chapter 11, and continues to write his own linking narrative here. Amulek had originally been called as a second witness to Alma, and Zeezrom will become a second witness to Amulek.
Chapter 11 ends with Zeezrom realizing his error: “and also Zeezrom began to tremble.” (Alma 11:46). Mormon tells us that there was more to what Amulek said, but that he chose not to write more. What he now chooses to write is Alma’s sermon to Zeezrom and the gathered crowd.