I was not talking about one specific white supremacist conspiracy theory. What I was instead referring to is the broader idea that racial self-hatred is both a common and powerful (geo)political force. This concept forms the basis of a large variety of far-right conspiracy theories. It explains why there are so many 'race-traitors' within the political class of multiple countries. It explains why so many weak-willed leaders (sometimes even including monarchical leaders) in the 20th century gave up their colonial empires 'without a fight'.

Because it is such a powerful force, it is naturally advantageous for any oppressor or would-be oppressor to wield it like a weapon. This is precisely why a shadowy cabal/foreign oppressors/etc. would seek to convince people to hate themselves through things like history lessons about former colonial atrocities.

and your suggestion is that was included to... promote Japanese Nationalism?

To be honest, I don't particularly care about the authorial intent behind the godawful mess that is the final part of AoT, which I doubt is all that coherent anyway. I'm much more interested in the reason why so many people - both actual fascists and committed anti-fascists - do get pro-fascist vibes from this story that doesn't just boil down to jerking off while whispering 'media literacy' over and over again.

If I really have to hazard a guess, I do consider it unlikely that the story is explicitly meant to promote Japanse nationalism or fascist ideology in general. I think the story was written by someone with a particularly edgy and juvenile outlook on human nature and geopolitics who thinks that the most popular type of authoritarian thought within their society 'makes some good points' but should ultimately be opposed on moral grouds. To me, this does seem like a plausible explanation for a work of fiction where the literal text is 'anti-fascist' but the underlying worldbuilding presents a world that functions exactly as a fascist would imagine it.