This is a fact, not a myth–at least, it is true in the northern colonies/states. Lower ceilings decrease air space and so concentrate the heat from the fireplace in a smaller number of cubic feet. That’s why in the South before air-conditioning, houses were often built with high ceilings, so the heat would rise and leave the lower portion of the room a little cooler. And in the North, before central heating, ceilings were often built lower.
Builders had all sorts of clever techniques to help keep a house cooler or warmer, techniques that are usually ignored or forgotten today when the thermostat instantly adjusts the temperature. To keep cool, they might site the house to face prevailing winds, put windows opposite one another to allow cross breezes, or build exterior fireplaces and chimneys rather than interior to dissipate the heat, build separate kitchens to keep the all-day cooking fires away from the main house, use central hallways with doors at each end to encourage a breeze, and build tall ceilings and large windows. As heating and cooling costs rise, we may well see a return of these techniques in new homes.