Author here. We do do the interviews blind to earlier screening results. That's clearly vital. But you're totally right that job performance is the real thing that matters
What kind of skill set? My previous company contracted with Galye Laakmann McDowell to coach the engineers before an acquisition due diligence. I thought it was unnecessary because we rarely used O() notation in our day-to-day. Since then, I'm designing a machine learning product and find sections in McDowell's book extremely relevant. My takeaway is that it all depends: are you hiring an architect or a hiring a construction crew? How do you measure such different skill sets? Or, how to measure with a job that combines both?