The Economist article referenced comes from 2016 and has no links to the studies it refers to. I will continue to look.
I am also highly skeptical about the quality of Grade School education in the US and see the revamping of curricula as a deckchair exercise.
I have a simpler explanation: If the math proficiency of the teachers is low - as it generally is based on SAT scores of Education Majors - why would we be surprised at the subsequent outcomes. To add to Economist author’s story line, if the swimming coach cannot swim, are you sure you want to have him or her teach your kids?