I agree with the overwhelming majority of what you say. I especially appreciate your stating that 10 people of the same skin color can be orders of magnitude more "diverse" than having the magic rainbow. This prevailing "wisdom" that if you mix the right % of people from different races, genders and ethnic backgrounds you get an ideal outcome is absolute nonsense. AA and quotas are also absolute nonsense. The entire idea that fostering "diversity" and "multiculturalism" will lead to improved outcomes is only a feel-good measure. There is absolutely no empirical evidence to suggest that a work force, student body, government body etc. performs optimally with the "right" mix of races and genders.
However, the article is citing an actual observable phenomenon. Children learn better when taught by a member of their own race. Young kids are not capable of rationalizing thoughts such as yours. e.g. "If the teacher is qualified, who gives a damn?" I can theorize about why this might be true, but would be called a racist, so let's stop at the point where the observed data end.
Yes, to hell with all of the "magic diversity formula" and other nonsense, but hiring black teachers for black students and white teachers for white students to achieve improved results apparently has some empirical basis, so why not do it?