My assumption is that there are a lot more people who are studying for these interviews than you think, which is why the proxy is failing (or producing homogeneous results)

On the other hand, it's okay to target sufficient but not necessary. False negatives can be fine, so long as you are finding enough true positives.

Even if you are systemically missing out on a sub-pool who are good, it's not necessarily a bad thing - particularly if you are realistic about its limitations. To my mind the real problem isn't that you might miss good people (you will) but that if your filter is too constrained the good people you find might be too homogeneous.