Rare only works as a qualifier when comparing it to some other probability of social mobility, either in an ideal or real system. The US has higher social mobility than most actual countries in the world and is pretty much middle of the pack in social mobility when it comes to developed (OECD) nations [0]
If I recall correctly, there was another case of a janitor working her way up to becoming a C-level executive at Microsoft and HP. Capitalism with a social safety net is what affords this social mobility because it doesn't matter what your credentials are if you can find a market for a novel product like Montañez did. You can argue about how much of a social safety net is required to prevent the poor from staying poor forever, but it's still capitalism that provides the impetus that lifts people up to where they don't need the social safety net anymore.
Compare this to a society where you're hooped if you're born into the wrong caste or without party connections and US social mobility doesn't seem that rare anymore.
[0]: https://www.oecd.org/inclusive-growth/inequality-and-opportu... figure 2