"10x" is just the wrong paradigm to explain the phenomenon. The person is not working at a throughput ten times higher than another person. It's more of an exponent that compounds over time. The best tech people can save a rapidly growing firm dozens of hires (or more) over time through better architecture, documentation, processes, communication strategy, etc. There's no way to suss this out entirely from an interview, but a smart interviewer can certainly get an idea if they ask the right questions.