This is pretty much spot on.
If OA had simply billed itself as Fantasy Japan, they would have gone a long way. I mean, IIRC, in the foreward to the book, one of the inspirations is James Clavell's Shogun. Which would be fine and would have worked.
But, they then took Japanese culture, language and various other bits and bobs, erased China and templated Japan over China. Given the 20th century history of these two countries, that's a REALLY bad idea. Never minding that Oriental basically skipped anything that wasn't Japan or China - poor Korea, Vietnam, Cambodia and whatnot get a really short shrift. HIstorically, the Khmer empire (what is now Cambodia) would fit much better in the sort of quasi-historical period of D&D. The site of Angkhor Wat was one of the largest cities in the world at the time.
But, yeah, it would be like taking a map of Great Britain, overlaying it with German names, German culture, religion and history and then calling the book European Adventures.
Not something that would fly.