Mormon had left the story of Amalickiah with his flight to the Lamanites. After that mention, Mormon turned to the aftermath of the story. At this point, he returns to Amalickiah, but to the story of Amalickiah among the Lamanites. Amalickiah manages to do just as Moroni had feared. He stirred up the Lamanites to make war on the Nephites.
There is no indication of how Mormon got the information about these actions, specifically that the Lamanites “feared to go top battle against the Nephites lest they should lose their lives.” From a purely historical standpoint, that seems somewhat unlikely and is certainly something that the Nephites would have said about the Lamanites, but much less likely to have been a Lamanite reason.
Regardless of the reasoning or methods, Amalickiah manages to become a commander of part of the Lamanite overking’s army.