I'm not sure I fully agree. Doing open source doesn't mean you do it well. You have no sense of how quickly, efficiently and independently they managed to achieve it. I'd much rather hear from prior experience, and probe about situations and scenarios they were in, projects and problems they contributed too, and hear the story of how they went about it, how long it took them, what they did in the face of setbacks and pressure, etc.

I have seen first hand developer that are just okay or below average successfully deliver on open source, because you have infinite time, no constraints, no stress and get to choose exactly what you do or contribute. But in a work environment they struggle, given ambiguous problems they struggle, given time constraints they struggle, given changing needs and demands they struggle, working within a team they struggle, given something outside their area of knowledge they struggle, etc.