Whiteboard weeds out candidates that cannot code period. I wouldn't believe the legend of the "senior engineer" who couldn't FizzBuzz until I met him. I've personally conducted interview where a simply word count function (take a string, count the words) couldn't be implemented in 30 minutes. And the resume listed proficiency in several programming languages.
Now this study tells us that competitive programmers aren't that great among the candidates that were hired, not among the pool of candidates. That's very different.