What does it mean to be "successful, controlling for socioeconomic status"?
Their success is defined by their peoples' socioeconomic status.
Their success is defined by their peoples' socioeconomic status.
Even Massachusetts would be superior than every European country per capita.
However, even if the US shows better outcomes when controlling for socioeconomic status, the distribution of people within socioeconomic status is relevant.
If the US has 1000 impoverished people and 10 middle class people, and Germany has 10 impoverished people and 1000 middle class people, the relevant comparison is the German middle class to the US impoverished class.
In other words, you'd want to compare outcomes based on percentiles regardless of socioeconomic status.