The sad part is that the generic generalist interview will have nothing to do with X, and if you need someone great at X-ing, might yield painful false negatives or even worse, false positives (since you can’t hire specifically for X).

I would argue that if you’re in a position to be picky you would want to hire experts in X who are also good generalists. One-trick ponies don’t do well after project X is finished and they need to contribute to other efforts.

A PhD isn’t really a one trick pony, but whatever. Even if that’s what you want (specialists who are good generalists), those interviews don’t really check the specialist part, just the generalist one.