According to the Migration Policy Institute, a research organization in Washington, there were 18 states in the U.S. in 2010 with an identical or significantly smaller percentage of foreign-born residents than Finland.
Second, the part you quote talks about variation between schools, and there definitely are Finnish schools with 0% immigrants as well as ones with 30% immigrants. Yet, a "remarkable lack of variation" in test scores.
Yes, the article claims Finland focuses on equality, but it also explains that socio-economic background is a major source of inequality and the Finnish schools have been able to diminish that effect.