Presumably this hypothesis is meant to explain why there is this observed asymmetry in the type of Neanderthal DNA we find in modern human populations that contain them, which is entirely autosomal. With none in the mitochondrial form, which is exclusively passed down along the female line, and also none in the Y-chromosome form, which is exclusively passed down along the male line.

Without weighing on the validity of their hypothesis that one or both sides found the other“especially attractive”, an alternative mechanism that could explain why we only see Neanderthal autosomal DNA in modern humans could be that only the female offspring of male-Neanderthal and female-sapiens pairings were reproductively fertile. This is more commonly the case in interspecies hybrids, see Haldane’s rule.

why no mitochondrial then?