Their surveying method is perfectly fine. The study is based on a cohort of kids who were born between 1980 and 1984 (in USA); and in 1980 Asian Americans were only 1.6% of USA population (http://en.wikipedia.org/wiki/Historical_racial_and_ethnic_de...).
It does disregard (relatively) recent immigrants in that regard, though; but if it's to be used as analysis of gov't policy/parenting/schooling/etc circumstances, then you'd want to look at people who grew up in USA policies and environment.
