I worked outside in too many cold winters to agree with this.
If I have my Arctic Parka, insulated coveralls, and snowboots on but leave my head exposed, most of the body heat loss is going through my head. I'm cold.
But just put a decent winter hat on, -40C is bearable--and I can get some work done.
I think that 7% figure comes from all areas of the body being exposed equally. That makes sense--even if all areas are prepared for winter.