Actually, it is outspoken very frequently in the Japanese dojos, at least the ones that I have been to. In fact, Matayoshi Kobudo draws heavily on several Chinese weapon styles, and Yamash-ita Sensei and his students readily admit it.
Of course, I don't doubt that there are other schools that try to claim that their art is original, and was developed solely within the bounds of Japan. I also don't doubt that these types of schools may be in the majority. I could easily just have been lucky to have the senseis that I have.
Where the Japanese and Chinese both become upset is when you suggest that they have a linked ancestry. That caused quite a stir between the two countries a couple of years ago.
And I will put in a third vote for it is likely that this is the (or at least a) source of Ju Jitsu. The ideas are very similar.