Anecdotally, it feels true. I, like many Americans, know an enormous number of immigrants and poor people that moved into the middle and upper-middle class through hard work and discipline. There is still plenty of opportunity to do that US if you apply yourself. Economic mobility is very high in the US (which is distinct from social mobility).

When I've worked in Europe, it has always been evident that this is much harder to achieve there for poor people and immigrants. The entire social system is setup to limit the ability of ambitious people to rise above whatever station they were born into in a way that isn't really a thing in the US. This contributes to why engineering wages are relatively low in Europe.