Just because you are prejudiced in favor of white people doesn't mean every other white person alive is also a racist.
The sad fact is, a correct assessment of reality is racist to some extent. Not because of skin colour, mind you, but because of confounding factors such as income, or self-fulfilling prophecies such as racism itself. Correlation is not causation, but the correlation is still there, and we can still draw inferences from them.
As for my personal assessment of the credibility of this story, I assumed the guy was white to begin with. If I believed he was black, I would have been like "of course", and marked off the story as even more credible. Because I have this stereotype in my head about police being racist. I'm now wondering to what extent this is actually true…
Does that mean he is a racist?