Anthropological: This passage may be more fully understood in the light of known aspects of Mesoamerican warfare. There are two immediately important parts of the description, first that the Lamanites have fled with a king wounded on the ground, and secondly, the king is wounded, not dead. While both of these facts are not necessarily foreign to modern warfare, they are more understandable in the context of Mesoamerican warfare.
First, even though the Limhites were not half so numerous as the Lamanites, their ambush is effective. While this is a simple enough explanation, we must remember that the warfare of the time was essentially hand to hand, not long distance. While there were thrown arrows (the atlatl) those might wound from a distance, but not necessarily kill the foe. Once the ambush had been sprung, the two armies now face off in hand to hand combat. In that scenario, even though taken by surprise, the net effect of having two Lamanites for every Limhite should suggest that even the ambush would not have been so completely effective as is described.
The key is the fallen king. With the capture of the king, the battle is essentially over. Under the conceptions of most of the Mesoamericans, the gods had decided the battle when the king fell. This is the same principle of ancient warfare that became ritualized into the game we call chess, where the end is over when the king is captured, regardless of how many of the "king's army" might remain.
The ambush was successful because it allowed the Limhites to break through to the king and "capture" him. This ended the battle, and the remaining Lamanite army might logically retreat from the battle so decided by "the gods." Notice that the Lamanite withdrawal is termed "speedy." While the modern mind might suggest this was a rout with the Lamanites fleeing in fear, it is more appropriate in the ancient context to see the recognition of the end of the battle, and the speedy removal before the manifest godly power of the opponent. There was no more reason to fight, not that there was no more will to fight.
Next, we note that the king was only wounded. While there were certainly casualties in Mesoamerican warfare, the nature of the weaponry as well as the general intent tended to injury rather than extermination. Thus it is not at all surprising that the king would still be alive. In the context of a Mesoamerican battle, it is even to be expected that there would be every attempt to capture the king rather than kill him.