100% of those that couldn't do it are not qualified candidates.
Some of those that could do it are good candidates.
When you bring in someone that has a nice looking CV and experience and recommendations and you ask them a very basic question and they can't respond at all, it is a safe disqualifier. It also doesn't take six hours.
Looks like you edited your comment. In any case, that's a non-statement, as some is quite different from nearly all (i.e. 90%).
OP stated nearly the inverse: that 90% of those who can do it are qualified candidates.
That is, your numbers are only valid if I accept the premise that I'm rejecting. I don't believe that 90% of people who can answer such a basic question are qualified.