Your post is essential. No one is claiming 100% of cops are white supremacists. One is claiming that it's sensible to assume they are.

If 20% of cops were white supremacists, and I was a minority, it would be sensible to behave as if every encounter had a significant chance of being with someone is looking to ruin my day.

The majority do not need to be unsafe for me to feel unsafe around the community. You have to factor in the potential power they wield (to kill you or take your freedom or seize your assets), combined with the odds that one will do it because they have wrong headed ideas about race.