The rout of the Lamanite and Amlicite armies is complete. The battle continues in their retreat, and eventually many of them die in the wilderness called Hermounts. To be devoured by the beasts suggests that those who so died would have been injured and left behind. In this area of the world there are jaguars that would certainly prey upon men, but which would certainly not take on an army.
Mormon's description of what happened to the army does not appear to be a first hand account, but rather one that is made of conjecture. His evidence for those who died by being devoured is the bones which have been found. When those bones were found is a question, but certainly it was not during the pursuit, but some time later.
What we have with Mormon's relation is second hand information and inference. The bones that are found are presumed to relate to this incident, but there would be little clear evidence for that fact. This type of "historical" recording is very typical of ancient writers. The supposition is easily taken as declaration of fact, a presumption that would not pass modern historical practices.
Textual: The 1830 edition does not have a chapter break at this point. While this is the end of the Lamanite/Amlicite army, it really is not the end of the story from Mormon's point of view. Textually, the next verses at the beginning of chapter 3 belong to this narrative unit, as it discusses the aftermath of the war from the Nephite perspective. Mormon's whole narrative unit shows the Nephites in peace, then a terrible war, and then the return to peace. The break at this point misses his intended conclusion to this unit.