I mean, kamikaze pilots did exist, so there had to be a certain level of what you're calling "brainwashing".

And unless it's also a myth (completely possible), but weren't there Japanese soldiers found on an island years after the war had ended who were convinced that it was still going on?

The "brainwashed" thing is somewhat true, at least from the perspective of an outsider, not due to a racial thing, but there is a cultural aspect in addition to the tendency for all sides to be brainwashed by their own propaganda.

But the Japanese propaganda told their soldiers to fight to the death, because if the Americans captured you, it would be worse than death. So, from the outside, they did appear to be brainwashed in that regard. Of course, Americans had similar propaganda making Japanese seem as evil as possible, often in the most racist way, so you'd have to say that Americans were brainwashed, too.

Also, culturally, I think American culture emphasizes each person more, while Japanese emphasizes community more, which means things like kamikaze are easier to sell. And that sort of thing also appears like brainwashing to the outside.