...core devs know how to use git, but that doesn't mean there aren't some decent coders that would be interested in contributing but have never used version control at all (it's sad but true)...
80% of serious version control users have never touched git. They have used rcs, cvs, svn, bitkeeper, perforce, team server, rational tools, revise, mercurial, darcs, what have you, but not yet git.
80% of git users are not competent git users.
Just estimates, mind you. Some of my best hires have been people who failed simple "obvious" tests. There's really nothing sad about it. There are a lot of people in the world who are not like you. It makes the world a better place. Many of them will be better than you at something extremely valuable to you. Some of them will be better than you at just about everything you care about.
Personally, when I have to work with someone, I put a pretty high importance on their not being a complete and utter rectal suppository. It outweighs things like coming from the right school or using the right text editor.