A racist is less likely to have black friends than a non-racist. I'd suggest that a racist is even less likely to have black co-founders than a non-racist.

edited to add: The reason the "look at my black friends" argument is mocked is that in many cases the friends referred to are not friends in the true sense of the word. There's no such distinction possible when talking about a co-founder. One either is or is not a co-founder.