But they wouldn't. They'd rank themselves something like 1,2,2,3. We're not dealing with a population collaborating to all rank themselves in order, but rather each person individually estimating where their abilities lie in the population.
The point is that if you ask someone in the, say, 5th percentile of ability what their ability is compared to the population, they might say 25th percentile. Ask someone at the 25th,and they might say 40th. At the 40th they could say 55th. And at the 90th, maybe they'll say 80th. So yes, if you order their guesses, they will be in roughly the correct order. But, crucially, that doesn't mean that they are ranking themselves correctly!
The point is that if you ask someone in the, say, 5th percentile of ability what their ability is compared to the population, they might say 25th percentile. Ask someone at the 25th,and they might say 40th. At the 40th they could say 55th. And at the 90th, maybe they'll say 80th. So yes, if you order their guesses, they will be in roughly the correct order. But, crucially, that doesn't mean that they are ranking themselves correctly!