The movie is You Only Live Twice, and believe me, the ninjas are not the problem with that movie. The hugely cringeworthy depiction of Asian women is. More's the pity, because it has dated an otherwise great movie.
As for accuracy - I'm not sure how that is the problem. Much of what we depict about historical figures is likely incorrect in some way. Again - I think intent is the real issue here - if you're dressing up to belittle and mock somebody or their culture, that's the problem.
The movie can have multiple problems and some of it is based on white privilege and the ignorance it breeds and encourages as if it's just okay to generalize non whites as James Bond films tended to do a lot (often just minions, as I recall)
Incorrect unintentionally can be accepted under the basis that we had ignorance of it. Acknowledging that is the first step to getting better at racial relations and cultural understandings.
One can dress up and belittle or mock someone with realizing it, and more often than not, it's white people that do so because they're either so lacking any kind of cultural heritage they latch onto the exotic and use it like it's in fashion/vogue, or they think they're entitled to it somehow because of how society regards that thing they dress up as just a joke (geisha costumes come to mind, not something you should do flippantly, which is part of Halloween costumes in general, I'd argue)
The British just got there first. Most cultures and nations have colonialist tendencies if given the chance. Japan's takeover of much of Asia was wrapped up in racial superiority, China has colonial intentions in Tibet and is busy displacing ethnic Uighurs with Han Chinese, Indonesia did similar things in Papua and Timor - there is a whole list.
I don't think people are denying that, it's just that, as I pointed out, it's more localized in nature, Japan didn't strive to go further until it became clear they had the power and then it backfired on them (WW2)
That's not to say what the British did was right, nor that white people in western nations don't benefit from it in some way. But that's a long way removed from someone putting a ninja costume on for Halloween because they think ninjas are cool. I think most people are able to discern the problems with true colonialism while having some fun on a holiday that is itself an appropriation.
Thinking ninjas are cool is different than appreciating them while also having that admiration. The fundamental issue is still the exoticism, the borderline tokenification of other non white races or making them seem like more than they are, often with historically inaccurate and ignorant ideas.
Is the holiday really an appropriation so much as a development of the idea in a different manner? That's like saying the German Christmas traditions were just ripping off Yule entirely rather than selective elements.
And a cultural practice by memesis that isn't making light of the culture itself or anything that is necessarily tied to a culture is distinct from costumes that are doing that by nature: "Oh, it'll be so fun to dress up as a samurai," while showing that you have a surface level appreciation in general of them or even understanding of the cultural significance of samurai.
Having humility is something I find that white people have major difficulty with in putting a mirror up to themselves and considering that htey benefited from a societal bias that gives them the benefit of the doubt and treats their cultural appropriation as somehow okay, while applying a double standard to any other racial group too often, as if they're crossing a line, but white people should be forgiven far easier for the same thing.
The distinction is a sense of genuine equality and equity with the cultural exchange, which Halloween as a practice doesn't really have, since the costumes are trivializing things, meant to make them less "scary" or such. Or did I miss something there?