First, Altruism aside, this will likely be a win for ycombinator. There are always opportunities to be had in targeting subgroups which others neglect.
And the problem with bias is that much of it is subconscious. Unless an organisation makes a conscious effort, it will likely discriminate.
This means there's an opportunity for YC to get easier access to the cream of the crop, merely by specifically recruiting. And articles like this are helpful too, as they'll signal to would be black entrepreneurs that they should not shy away from YC.
Second, this could be very important for future african-american entrepreneurs. Reference examples are so, so important. I (white male) did not become a programmer, despite a love for math and computers. I literally didn't have a single programmer in my extended family or circle of close friends. I played around with computers, but I had no idea what a programming language was until age 25.
(If this sounds incurious, think about what massive blind spots you yourself had until recently)
If YC can fund an african-american entrepreneur who succeeds massively, then this will inspire thousands of others.
The same arguments apply to recruiting women, and inspiring women. I applaud this move, and YC's earlier initiative to recruit women.