Yeah, I think "hybrid vigor" is probably nonsense. However, interracial couples tend to be left-leaning, which also implies fewer children, so I don't think it makes sense to infer causality (or lack thereof) the way you are. Too many competing factors.
At a minimum, the study suggests that the fitness golden mean between inbreeding depression and outbreeding depression is actually much closer to the intraracial side of the spectrum than the interracial.
