If you define socioeconomic status narrowly (gender, race, age, net wealth), then I was wrong to say that the success of European countries is rooted in their socioeconomic status. I'm happy to take that point back, and focus on why I don't think controlling for socioeconomic status is the right analysis.

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.