“White Americans” is very much not a “random group defined by subjective criteria”. Self-identification as white is extremely highly correlated with objective, measurable metrics like percent of European ancestry, and this also makes US whites directly comparable to population of European countries, which as it happens still are overwhelmingly comprised by people of European ancestry.
I am not selecting best performing group for the sake of comparison, I’m just comparing various natural and obvious population clusters that have been understood and distinguished by everyone completely unrelatedly to the discussion of educational outcomes. The category of white Americans has not been invented to show how great US education is. I am totally interested in comparing educational outcomes of white Americans with other major ethnic or ancestral populations in other countries, it’s just white Americans come out ahead almost every single time.