North Koreans and South Koreans are the same race, divided only by fairly recent politics. This is nationalist, if anything, not racist. Try replacing "North Korean" with "Soviets".
"Don't trust him, he's from Israel"
"But it's true!" is in my opinion a poor response to accusations of prejudice. "Black people are criminals" "That's racist" "But it's true, look at the statistics!" It doesn't make the original statement any less racist.