Here's the thing about that joke: you could tell it
no matter what the standards of people at medical school, and
no matter what the difficulty of passing the examinations.
If medical schools are good at accepting only people who are likely to make good doctors, and at failing students who turn out not to be good enough after all, I'm not all that bothered by the prospect of being treated by someone who just barely passed. (Of course that's a big "if".)
The same goes for the OP here. How do you define that ten-point scale? If, e.g., "6" means "doing a good job, but there are other better people around" then firing your sixes may be a lousy idea: perhaps all the better people are much more expensive and you get better value from a cheaper 6; perhaps all the better people are off working for other companies; perhaps in this particular job the difference between a 6 and a 10 doesn't really matter all that much, and the cost in recruitment effort and training will outweigh the benefits of firing your 6 and getting someone better; perhaps the person currently in the job is a 6 who's working his or her ass off to turn into a 7, then an 8, then a 9, then a 10.
... Or, of course, perhaps not. The original article does specify that it's talking about people who "cannot do the job". But what sort of scale are you using where 6 means "cannot do the job"?
I think there's something badly wrong with the idea that Only The Best Will Do. Because (1) for a lot of purposes, someone who's not The Best can still be plenty good enough, and (2) there aren't enough of The Best to fill all the jobs, and (3) if enough companies start firing everyone but The Best you're left with a large number of people starving because no one is willing to employ them to do the jobs they could do perfectly adequately if employers weren't all insisting on getting The Best. Oh, and (4) guess what, these companies generally aren't keen on paying every single employee a superstar salary.
There are jobs for which you absolutely need the very best people you can get. In, for instance, a new startup, that might be all the jobs you've got. In that case, by all means insist on hiring only the best people and firing anyone who turns out not to be. But in that case, you'd better also be offering rewards commensurate to your demands.