She is half japanese because half of her family and genetic history is from japan. The other half is from her african american father. Isn't it more bigoted to deny that half of her comes from her african american father?
Also, there is a difference between ethnicity and nationality. She may be a full japanese national, but she is not a full ethnic japanese.
Just like charlize theron could be a south african national, but you wouldn't call her a black woman.
I agree with her that being a japanese national should have nothing to do with heredity or skin color, but heredity and skin color most definitely has something to do with the japanese ethnicity.
To claim otherwise is to claim there is no suching thing as a japanese ethnicity and it destroys any sense of human diversity that we all pretend to love while trying to actively destroy human diversity.