It seems more like a prisoner's dilemma situation, where hiring from an in-group only works if you trust other members of the in-group enough to return the favor. Otherwise they could "betray", hiring a better candidate from outside the group who will perform better in the role. The person betraying would get both a better employee and the social kudos of having a diverse hiring record. I'd expect the trust strategy is a lot more common among fraternities, religious groups, diaspora communities, etc where community bonds are much stronger.