Not just optically. Even slightly different cultures can result in novel ways of thinking or world views that bring something new.

Diverse viewpoints are more likely to arise from introducing 3 black women to a group of 3 white men than if you introduced 3 more white men.

> it is quite possible that yet another white man with a different background would provide the greatest diversity,

Are you and the parent really arguing white men are even better than black women at diversity?

People with different backgrounds are "better" at diversity than people who look dissimilar, for sure. The wealthy black woman who went to school at Stanford alongside all the other white men on your team does not bring any meaningful difference in viewpoint. The tribesman from Kenya, on the other hand, comes with a very different outlook.

Of course, something akin to "Must be a US Citizen" is attached to most jobs because we don't actually care about diversity, just optics.