You're making a mistake.
Go back and read my post again. I did not confine "homogenous" to race, and spelled that out quite clearly.
African cultures are intensely tribal and xenophobic. That they are all the same color is totally irrelevant. Same for many other cultures around the world that are ethnically the same, but culturally different.
You're "exception" is actually a proof.
I am not claiming that tribalism and xenophobia are the only factors human violence. That would be ludicrous. But it is a big factor, and it goes a long way to explaining how the relationship between gun ownership and violence varies drastically around the world.