It does sound like a classic case of " This encounter was meant to be fought in the dark, lets fight" thinking by the players as opposed to sane options like retreat and regroup.
Totally. That's why it sounds like such a great encounter to me.

Players need to "think outside the box". If they don't, their characters should pay the price. And light sources are critical! I'm amazed that these fellows stood around in the dark and fought it out. That has to look like a turkey shoot if you're a monster.