I understand your points, but you need to understand that this is not just a matter of underrepresentation, but of oppression.

Why would blacks have high representation in the first place?

What I said was that black people are not really represented in anime, not that they always should be more represented. If a story takes place in Japan, I don't think it would be that big of a deal, but if it takes place in a different location in the world (or one that is meant to mirror a place in the world i.e. Hizuru being Japan) that has a higher percentage of black people, then black people shouldn't be nonexistent.

Second, how would giving the role to Boyega give the character better representation?

Because he is black. Giving the role of black characters to black people would mean that there are more black people in the business. It's also worth talking about how this is AOT, one of the biggest franchises in Japan, and possibly in the world. Doing something for diversity could cause this to spread like the comic book industry, which started out showing a little bit of diversity, and is now incredibly diverse.

Casting the role based on superficial similarities like skin colour seems somewhat racist, no?

This is completely the opposite. If MAPPA is unable to find any good black/blasian voice actors then an asian or white person should voice Onyankpon. However, if MAPPA is able to find someone of color is talented, then they should be given the role, as it would mean for a more accurate character depiction, and increase diversity within the industry. Attempting to bring more diversity into an industry, and thus breaking the normality of racism and xenophobia in it, is not racist one bit. Race is also not superficial in this case. Speech is something heavily determined by race, and in voice acting, being able to connect with your character in ways helps create a better performance.

You shouldn't just assume western cultural dogmas are validly applicable to non-western countries.

I did not. You did. Here is an article that says that not only are white not oppressed THEY ARE PRIVILEGED. Here is another article, that not only says that black people are not treated as well as white people, but that they are oppressed and discriminated against.

(And as a side note, even if whites are treated better in Japan than blacks, whites would still be a minority population facing discrimination

Being a minority does not equal facing discrimination.

Yet both minority groups get representation stressed. So therefore, to be consistent, the entire Japanese AoT cast must be recast with whites.)

If the reasoning behind white people not being represented within the Japanese voice acting industry is their race, then characters who are meant to be white should in fact be voiced by either mixed, or pure white people.