Pasi Sahlberg goes out of his way to emphasize that his book Finnish Lessons is not meant as a how-to guide for fixing the education systems of other countries.
What the article says is you can't use the differences in immigration to blanket-deny all other findings. The US and Finland have their socio-economic issues but the Finnish school system has been able to diminish their effect on education outcomes.
The only lesson the US can really draw from this is that if we reduced the number of immigrants we have, our average scores would go up. So what? Raising averages due to composition changes is pointless.
Nowadays, Finland has much of the same rate of immigration than US. Historically it has not been so. But today, around 20000 people move into Finland every year. Whereas the US gets a bit more than one million new inhabitants. Finland has 1/60th of the population of US. The performance is still better, though there is the same ratio of first-generation immigrants.
And I'd guess a lot less people relocating around the world know Finnish than the dominant teaching language in the US...