You don’t know that.
All you know is that he hates Blacks. He may fly the flag for entirely different reasons.
Well, he's still alive, so thankfully we can ask him if was just coincidentally waiving the confederate flag around while preaching hate speech against blacks. It's also theoretically possible he was wearing the British-colony era Rhodesian flag for reasons other than preferring the country under white rule. Maybe he just liked the design and colors of the old Rhodesian flag better than the current Zimbabwe flag, and it had nothing to do with his feelings about segregation).
I suppose both are possible but I wouldn't bet on it.