If you count Khan Academy and MIT's OCW, sure the Anglo-sphere has unparalleled resources. But a minor point of the article is how the way Americans are taught in conventional public school is just to do the basic work. Math is treated as a hurdle to clear rather than the lifelong mental sport.
But here's a point I would have liked examined further in the article: where does one find material or instruction that would give you a competitive structural thinking needed to grapple Olympiad or Putnam-level content beyond enrichment courses? In China, the best students are scouted early by teachers and prepared by the Chinese government through special camps and their own difficult examinations. I'd assume the same thing happened/happens in Russia. In the United States, by default (and probably by design), its difficult to know on what level a student can stand in regards to everyone else until the very end. At least in a public school.
Parents and students can use proxies like how early AP subjects are offered, USA today rankings, or the success of past alumni. But, unless you're in a well-endowed private schools, magnet/exam schools, or private enrichment program like AoPS (Art of Problem solving) or Talent Identification Programs (many of which are now shutting down) there are few ways to know how well the instruction one receives as a precocious and motivated high school compare to those of the best among the nation or the world.
One could argue if you have to wait until high school to understand where you stand, it's too late. But that's my point, many students may have resources and talent but the lack the expertise to deciding where to and how begin in those crucial first years. Few states have strong "gifted" programs and the one's that do aren't really any better than "normal" instruction in China or Japan.