In the movie, the Huns had attacked China, but they never did. Prussia attacked China, NOT the Huns. I don't think this slip-up was for the sole purpose of Mushoo's line, "Let's go kick some hunnybun!"
Actually the Huns did attack China. Back then in that area, they were called Hiong. The Great Wall was built to keep them out. I don't remember if the movie made reference to Attila or not. If his name was mentioned, it would be fallacious to call that slip up since it is very possible that the Huns pushed West after being defeated or encountering difficulty in the East. Check out Thompson's book about this subject. It's good.