I fail to see the problem about it, the joke was about both Koreas, who were mortal enemies since their birth, ganging up on a common enemy, Japan. Like they say they are so different, when in fact that deep inside they are just the same.

It's true, it is quite stereotypical of the asian nation, and maybe blatantly racist, despite that, let me remind you that you are in no way to judge, since.. Well, all of us are neither Japanese nor Korean.

Why so sensitive over something so minor.