Not to say that we shouldn't care about average competency, but these kinds of studies that explicitly or implicitly portend imminent doom for the US miss the mark.
I wonder what's better for the economy of a country as a whole - a high number of moderately well educated people or a good number of extremely well educated people but a high number of poorly educated people.
The analysis of this study dovetails with other analysis of PISA rankings, about how the top 10% of students in Japan and Korea have significantly better math proficiency than the top 10% of math students in the US, how their top 20% is better than the US's top 20%.
This is what the data says and the analysis of the data. The typical reaction is of your type - you don't even read the article title, assume it is saying average rankings are below, and repeat the Panglossian idea that the US children of the well-educated who are ahead of most US students are on top of the world, or near it. The data shows otherwise. Even non-stringent observation shows otherwise, if you take a look around at who is getting doctorates in mathematics and the like at top American universities.