Interesting, thanks for sharing. You're right about the folklore, and while there are some strong outliers that have done quite well in the US, but those would be hard to attribute to anything special about US education and support systems.

Although the ranking is named "social mobility" it isn't clear that that is what is measured; the report seems slightly confused.

The visual capitalist provides a definition of social mobility - children having a better life than their parents or being unconstrained by their socio-economic status. But the index they reference doesn't measure that. It measures more general Quality of Life metrics like prevalence of malnourishment or quality of the social safety net.

So while a high score reflects an easier life for the low end of the social ladder it doesn't actually measure their ability to move up it. I could on paper come up with a country that scored quite well on most of the index measures and had a rigid caste system where children could never outperform their parents and cannot escape their birth status.