How do you know? Because they failed your whiteboard test? That doesn't necessarily tell you anything about what they can do with a real workstation and realistic deadlines, unless you've empirically evaluated its effectiveness (including hiring some people who failed it).
This isn't very scientific, but I feel pretty confident about not hiring the people I described. Frankly, I have no interest in working with someone who can't come up with the syntax to define a function in the language of their choice. This is a real thing I have seen from people with significant experience on their resume.
There are candidates who are more on the line, and those I'm less confident about rejecting. But we can't just hire the hundreds of applicants per position we see, and I haven't come across a filtering strategy that isn't obviously worse than what we do now (which, to be clear, is the candidate's choice between whiteboarding and coding in an editor).
