I think that neglects how much children can help their parents learn the host country’s language. There are several variables involved, as the parents doing well economically will also help their children in a different way.
If you don't keep poor people suffering like folks do in the US, being poor isn't as big of a detriment and the adults are perfectly able to improve on their own time. And it isn't like being an immigrant means you are poor.
Personally I found the 80% figure hard to believe. According to something else I saw online, it was 20th percentile in the US = 50th percentile in Europe. Which still shows how dominant the US is!
[0] https://www.washingtonpost.com/graphics/2018/business/global...
Other differences between the USA 20th percentile group and the Euro 50th percentile group is that one of them has healthcare and a longer lifespan.