To be perfectly upfront with my own beliefs, I believe economic disparities is the mother of all other disparities and that many of these statistics that show disparities in outcomes for whites vs blacks are just echos of that economic disparity. That, I think, is the core disparity and the core way in which "the system" oppresses people across generations.
That being said, I don't think gender is a good counterpoint given that biology does have a significant role in some disparities in outcomes for men & women (for example life expectancy). With race, as cited in the letter, biological explanations aren't enough to explain the differences.
> Why is it so popular to talk about these same issues as systemic oppression against group X (relative to group Y), while ignoring or the fact that those same issues affect group XY, often to an even greater degree (relative to group XX)?
Speculatively, with respect to blacks in America, I'm going to guess that it isn't any single issue that is concerning, but the totality of them. Yes, a given issue X might affect group Y more than it does blacks, but blacks in the US as a population are affected by multiple such issues and I can't think of many areas where they have a leg up over other groups.