Coates is pointing out that there are good reasons to suspect strong sample bias. OkCupid didn't address this -- they just assumed an unbiased sample. I find Coates' argument far more compelling than OkCupid's blind assumption.

Blacks do look seriously underrepresented based on the response rates to the two questions. (Suggesting that blacks account for around 4% of OkCupid's users.) That's hardly proof of anything, but it does offer some credence to Coates' argument.

In the real world, it's often very hard to find compelling data, so we sometimes have to rely on compelling argument.