Decode's alternative proposal is that the flipped version was carried for many years in a different human lineage, one of the archaic populations that preceded the emergence of anatomically modern humans in Africa 150,000 years ago. Then, in some episode of rape or interbreeding, a single copy of the flipped version entered the modern human lineage some time before humans left Africa 60,000 years ago.
Seems to me we have evidence of a "cross" in Europe at a later time, say when Neandertals and Cro-Magnon people co-existed.
The far smaller incidence of this genetic lineage occuring in African or Asian populations is probably due more to the romantic failure of Western European adventurers than any other factor.