They hired the candidate as contractor on a one-week term. He paired with the senior members of the team and worked on the code base from day one. He started as a full-time employee two weeks later.
Another thing is that companies like Google or Facebook can't seem to afford this, they prefer false negatives, bad for them.
The fact that Google produces "pretty decent" products doesn't actually tell us whether their famously stringent comp sci trivia quiz system produces any better results than, say, throwing a packet of resumes down the stairs and hiring the ones that land face up.
You suspect that one could get the same results by throwing resumes down the stairs and hiring all the ones that land face up. That is a testable hypothesis. I recommend you test it and post back here with your results. I look forward to reading your report.