Keep in mind that is at a particular price point.
NVIDIA's top of the range chip is ahead of AMD's, and the 3080's SKU is at a lower binning point on the bell curve than the 6950's.
Hence NVIDIA would be able to maintain a performance per watt crown at the 6950's price point if it sold its highest bins cheaper.
Given the gap in transistor density, that is an exorbitant architectural delta.