I think it's more like, people feel more comfortable around groups that of the same culture / social class. I'd certainly feel more comfortable around middle-class college educated black people than I would around some hooligans from Glasgow despite being of Scottish extraction. Unfortunately in the US we're pretty segregated by race, so it's often an effective proxy for culture / class.
You have anything to debunk other than some snide remark. Yes, people feel comfortable in their ethnic groups, that does mean they do not have friends outside of it or inter-marry. Look around yourself, and spare me of snark.
You've almost figured it out.