Ah, right, thanks! That wasn't actually part of the statement they quoted (/posed within quotation marks), whence my confusion.

Edit: Or is the point that 20% of floor workers in total do become managers, but that's precisely (or very strongly correlatedly) the subset containing those who have degrees? If so, that makes a lot more sense. I don't really think it's fair to describe as a fallacy, though - it's a function of the philosophy of probability, which doesn't describe the world but our knowledge of it. You could make an analogous statement about roulette wheels, with perfect knowledge of their physics.

Yep, a population statistic isn’t predictive for an individual if the population is heterogeneous. And in this case, designed to be heterogeneous.