This is an odd use of of the term caste. Or even class. Both of these imply low or no mobility within social hierarchies. But I don’t think this is the case. “An Outer Party member could reach the Inner Party by chancing into an early job at a lottery-ticket company (such as Facebook or Google) or by becoming a successful entrepreneur. But that’s rare; most of the Outer Party prefers working for the Inner Party, grad…
What the author is saying is "most people in the Outer Party will not join the Inner Party", not "most people in the Inner Party started in the Inner Party". Similarly, you could say it's rare for a US citizen to become President, even though all Presidents were US citizens.