I used Lisp and Python as examples because I don't know C myself. In any case, the point was that when people say they know a technology, they might mean that they can answer any question you can come up with, or they might mean that they once knew it, and can become proficient with it again in a short time. Of course, some people don't actually know anything about it, but I doubt this number is quite as high as 80%.
I can believe the 80% number. I don't know any formal studies (surely there have been some by now?) but I've heard and read a lot of anecdotes (like the one in this thread) from technical interviewers about how a high percentage of their applicants can't write code or are in some other way incompetent for the job; I have yet to hear from one about how almost all their applicants are awesome and they wished they could hire them all. If you haven't seen the accounts about the FizzBuzz problem alone (and the hilarious failures of people online with no stress or time constraints pasting their incorrect solutions in blog comments to prove their skillz) it's worth half an hour of your time to read about it.
