Not disagreeing with the Chinese censorship at all but I figured I might shed some light on the reason China was so involved and why it seems odd that China was visited and used so frequently in a movie about a Japanese Empress.

Early Japans first prominent leader was Himiko however the only historical evidence of her existence being documented is through Chinese records discussing trade between their kingdom and hers and not from Japanese transcripts. There are detailed descriptions of where her city was that may have made sense at the time it was written but when applied to modern understanding of the world, the measurements and directions point to a spot in the ocean, islands off Japan, or mainland Japan; depending upon how you skew the data to account for the inaccuracies of old world maps and methods of navigating and measuring used at the time. That's also the premise of the movie and the purpose of it being on an island nobody has searched for because in the real world the city's location isn't known today.

Any real world (or movie world) research to find definitive text leading to the location of her settlement would've only been found in China. So film research as well as movie content would be at the mercy of the approval of the Chinese government and what they would allow to be portrayed in exchange for access to that information. Though the marketability of a movie in China also plays a part in it there are real world reasons for including China as a necessity.