No, they are saying the blood types of different human (sub)species were incompatible so many mixed children would have their red blood cells attacked by antibodies from their mother, resulting in hemolytic disease of the newborn. Think rhesus babies.
This means when these human populations mixed most genetic material from the Neanderthals would die out because it would also come with the high risk of dying from hemolysis.
So modern humans outperformed Neanderthals in many ways and when the few remaining Neanderthals bred with the modern humans most of their offspring died.
