The part that makes the least sense here is that supposedly the grammar hasn't changed in 600 years. I'm not sure about Japanese in particular, but the idea that a language's grammar has stayed constant for 600 years is pretty insane. Real life Japanese had a major influx of foreign vocabulary in the past few hundred years, had a bunch of science terms invented when western books were translated to Japanese, had its character systems simplified, had hiragana and katakana essentially swap places...