ancap thought is perfectly compatible with a privatized police force

Are you referring to a private police force because of the mention of the Thin Blue Line flag?

ancap thought is perfectly compatible with […] white supremacy

It depends what you mean by "perfectly compatible". An ancap would believe that the state shouldn't be able to prevent a person from being a white supremacist.

And the Gadsden flag is commonly understood as being against state interference, not other forms of oppression.

I agree. Perhaps my previous comment was lacking in clarity.

Edit: and while I agree that it seems like an odd combination, there are really plenty libertarians out there who think they need to buy out /bribe the local police force to get them on their side should things go their way.

I would be hesitant to refer to such an individual as a libertarian. At the very least, not without further information.