Moroni knew where the prisoners were and decided to take them without an exchange. To do so, he creates a ruse that will make the process easier. What he requires is to “find a man who was a descendant of Laman.” He finds one who was one of the guards who had to flee when Amalickiah’s men killed the Lamanite king. The simplest reading of this ruse would be that Moroni was looking for someone who looked like a Lamanite and who had a dark skin.
The actual event proves the opposite. There was no visible difference between the Lamanite and the Nephites who went with him. If there were one dark-skinned man accompanied by several visibly “white” men, the ruse couldn’t have worked. It is therefore much more reasonable that what was needed was someone to speak in a language and perhaps accent that would be convincing. Indeed, it was only the man named Laman who is depicted as speaking.