This points to the interesting relationship between the rhetoric of human rights and patriotism. How do we reconcile calling a nation like the one DuBois just described a 'great nation'? The U.S. is not just guilty of segregation, there are a plethora of other aspects of its history that contradict the idea that the country would be "in the forefront of the march towards peace and democracy."
Yet at the same time, the promise of a country that doesn't (and will never) exist is a common trope, almost an appeal to a better self. I wonder how much of this was deliberately targeted at the UN, an organization, where the US has exercised hegemonic soft power for decades, and thus a narrative of US excellence might ring true.