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I think they mean it probabilistically: given the cost, a "stupid and lazy" person is less likely to get a degree, so employers can hope that the percentage of such people among university grads will be lower than in the general population.

Start with “the market for lemons” by George akerlof.

https://en.m.wikipedia.org/wiki/The_Market_for_Lemons

The case you are confused by is a version of the model there. The relevant material is covered in any intermediate-to-advanced microeconomics textbook.

on average the "cleverer" ones get the degrees easier, hence there's a higher chance to pick a "cleverer" candidate if selecting by this criterion than when selecting at random