I think that's a bit far-fetched. It's JKR's world, after all. If she wanted to make Kappas Mongolian, or Estonian for that matter, there's nothing stopping her. Just because a bunch of ignorant Muggles think they're Japanese, doesn't make it true, after all. :-) No, I think she intended Snape to be wrong, as a little in-joke at Snape's expense for trivia-minded readers to catch. Then, in FB, she turned it into a mujch more obvious joke so that the non-trivia-minded readers could get the joke, too.