Black American lady here, it would have been nice to see Native Americans here, they also face much discrimination and poverty. Obesity rates are generally high. Black men are fitter than most American men and black women are statistically only slightly larger than the average American woman (the media exaggerates this misconception). I grew up poor and remained poor until recently, and I lived in an extremely racist place.
I ate very well, was a size 2, worked out and slept well, but there is only so much you can do when you face extreme obstacles. This study should be replicated but adjust for income and then compare the results. I think racism is just part of why this is a thing, lack of money is a huge stress factor and stress literally shrinks the brain which can be permanent if the stress isn't resolved. Black people are more likely to be single and that can also affect the brain, but higher income increases the odds of a black person getting married or having a college degree for black women. Being poor is generally associated with faster aging and premature death.