But there are plenty that are, or use some well known Chinese legend or element.
And so much cultural mash-up. Very, very good mashup.
I don't think it is the same thing... Japanese culture grew up in the shadow of China. They got their writing system (one of the main ones anyway, and the first one) from China. They got one of their major religions from China. They got a lot of stories, traditions, and attitudes from China as well. So, when you look at elements in Japanese fantasy, or even some myths/legends, a lot of it is heavily influenced by Chinese elements. Consider the whole thing with 'Fantasy Martial Arts' (which covers a lot of what makes up 'superhero' equivalents in both Japanese and Chinese lore and fiction) and the associated teaching traditions and attitudes about how and why you pass on knowledge, its all very clearly got a pretty common source.
The point is, if you see something in a Japanese Manga or whatnot, just because it evokes something that is typical of Chinese stories or culture, doesn't mean it isn't very much a traditional part of Japanese culture. Same with Korea, which has a very strong tradition of getting things from Chinese culture as well. Its a lot like European cultures, actually. King Arthur and the whole Arthurian cycle as it is generally known to us today (IE Mallory as the primary source) is a mashup of Welsh, Irish, British, and Medieval French which is mashed in on top of that, and then transmitted BACK to England. That doesn't make it not a British legend though. You can see the elements that came from various places, but they do all kind of just belong together. It is kind of the same with Tolkien. He was drawing MOSTLY from Old English mythology, to a degree, but when he mashes in something that might be referencing Finnish myths, its hard to say where one starts and another one ends.
So, I think Japanese Manga and such are pretty uniquely Japanese, they aren't appropriating stuff from someone else, its just that their culture drew a lot of influences from another one.
IMHO, as others have stated, this is a lot of the issue with OA, it just doesn't seem to understand. They take a uniquely Japanese (and rather historically narrow and anachronistic one at that) and then view all of Chinese culture as if it was basically the same. Actually a lot of it COULD work, but not without a lot more subtle presentation. Like, Chinese ideas of honor maybe do sound a good bit like Bushido, but you cannot just lump it all under this one term and not do a lot of violence to the whole thing. OA's 'not-China' does not read much like China at all in many ways (though in other ways maybe it does capture some of the general concept). Anyway, I'm not really qualified to say a whole lot on that subject, as I probably know no more than Zeb Cook, lol! (not to knock the guy, he seems rather cool).