Translated to mathematics, the fact that a white person has black friends, is considered an evidence for hypothesis that the person is a racist.

That's a bit of a bad faith interpretation; I see it as meaning something more like "Having black friends is not sufficiently strong evidence to push you out of the 'racist' category".

A bit as if I spent all day laughing and pointing at ugly and disabled people, and when someone called me an asshole I replied "I'm not an asshole, I helped an old lady cross the street last week". Even assholes can point to examples of nice deeds they did, even racists can point to black "friends".