Metrics that can be gamed can still have a lot of signal in them because the cost of gaming the metric is higher than the benefit of gaming things.

There's a branch of game theory/economics called signalling theory that's applicable here: https://en.wikipedia.org/wiki/Signalling_theory

Also, if being able to learn completely arbitrary things is correlated with intelligence and correlated with job performance, then people who are willing to jump through that arbitrary hoop are telling you that 1) they think they're more capable (faster learners) than average 2) they're willing to put in the effort to prove it to you. Maybe you're not a getting a pure signal of mathematical ability, but some combination of drive + ability + learning fast, and you care about all three.