Childish? No! I call it healthy competition. See where our country is now because we were always thinking 'beat Japan, beat Japan' as our standard of success. Now we are far ahead of Japan in shipbuilding, and we'll catch up elsewhere too!
As for the Japanese 6th gen aircraft thread, the article posted there was also talking about the KFX. The Korean thread starter just ignored the Korean bits to troll about 6th gen.