I think the Japanese probably started cremating with the introduction of Buddhism. It's actually a Hindu practice, but of course Buddhism picked it up as well. But it never quite caught on in China. Part of the reason this is that the proper burial of the body is tied up with all the earlier animistic beliefs and ancestor worship that was practiced by the people populating and Yantze and yellow river regions.

I guess artistic license is fine. It's more like it just kind of threw me off the story. Here it was, going along quite beautifully, and out pops out this line about ashes of ancestors, which would kind of shock any Chinese familiar with history. And suddenly I'm more preoccupied with burial practices than the story itself. But of course Yue and Wu are Japanese now. I'll keep that in mind. :P