That has got to be one of the most sweeping and misleading statements of all time.
DEHYDRATION IS ONE OF THE MAJOR CAUSES OF HYPOTHERMIA!

Your body needs water in order to process it's on board fuel load. That's the food in your stomach & the fat on your body. No water, no fuel. In below freezing conditions you dehydrate as fast as in a high temperature enviroment.

One other point: The Rule of Three's is a guide, not a hard and fast set of priority's that change according to the situation. It's like the Breathing, Bleeding, Burns and Broken Bones taught in basic first aid.
Don't get fixated. It WILL kill you.