It's one data point in a pretty large body of evidence; the FBI thinks they're infiltrating law enforcement in a widespread fashion.

A fascinating study from Stanford looked at police traffic stops nationally around the daylight savings switch (as a natural experimental control) and found pretty hard evidence cops treat black drivers very differently during the day (i.e. when they can see their skin color).

https://news.stanford.edu/stories/2020/05/veil-darkness-redu...

Additional aspect of this: "you're a white supremacist" is almost certainly a First Amendment protected statement of opinion that can't be defamatory.