Maybe, but how many high-performing Asian (or other) kids in the US are going to Kumon/the Mathnasium/other tutoring services (or getting parental help)? At least in the American case, we can be reasonably sure that a lot of those kids are getting their knowledge from sources
outside the schools.
I have a couple dozen random math books in my house (my analysis stuck to four Algebra 1 texts). I've also looked closely at every book my kids have brought home over the years, and I can say without reservation that today's mainstream US math books suck eggs.
I don't know about Japanese math books because I can't read Japanese. But my eldest went to a French school that used the French national curriculum. The math books he used were superior to the American books (I still have them). The Irish mathematics curriculum is also high-quality. Certainly, at least some students in those countries get extra help, but that fact doesn't diminish the quality of the math education in those places.