Another plausible bias theory might be that they are simply old and extremely high status and that high status old people rarely make major updates to their beliefs in the absence of compelling evidence even if they are very intelligent. It could be that there was a consensus in favor of genetic differences in intelligence when they were young, and that there is now stronger but not overwhelming evidence against that old consensus, plus strong social pressure. Who's likely to not move in response to that evidence plus social pressure?

Yet another bias theory is that Watson just likes to be outrageous and see what he can get away with. I've read "The Double Helix" and it encourages that interpretation..