Even as I am not a history buff, I know damn well there was more than just 300 men fighting against the Persians at Thermopylae.
Even the movie 300 showed how there were other forces fighting else where from Leonidas and his men.
But then again, the "myth" of 300 men at Thermopylae was more of a "tale" than a "myth".
It's clearly passed down as a tale to boldster courage and rally people for a war. And it got so famous people thought it was real without thinking of the biases clearly shown, exactly because it's an impressive "tale". Which is also shown as such in the movie too (the whole event was presented as a motivational talk by one of the messengers who was leading the battle at the end of the movie).
So yes. The guy, who, if I'm not mistaken, was not even Japanese, and only knew it from some convenient Japanese memory can be justifiably talk about said "tale" as if it was true.
I have already put up with the shoddy excuse for convenient knowledge as him having the memories of some modern man and Hijikata Toshizou as well as Jeane d'Arc showing up as nothing like their actual historical counterparts. Might as well just let the inaccuracy of a battle "tale" go.