Genetics? The article doesn't even pause to consider the possibility that Finnish people, a small and relatively isolated population, might just happen to be, on average, genetically just a bit smarter than world population at large. While thinking about these things is horrendously unfashionable, it should be considered before we start assuming that we can replicate their success by copying their school system.
Equally unfashionable is the idea that there are advantages to homogeneous societies and classrooms.
Education in the U.S. is currently deficient, and we should look at why and how and try to find out what works. Unfortunately, it's tricky to run experiments in education, because of political considerations and largely because the result has real effects on people's lives, but it's what needs to be done to improve.