I'm not sure I'm following, do you mean that the company may not be extracting optimal performance out of them? I'd agree, great people don't always do great work, often simply due to the environment they're in.
Having said that, i think when people have a great personal Github repo where it appears they should be able to answer the above question, but in practice cannot do it without looking up the answer on Stack Overflow...then there is an issue.
Not trying to be a smart ass, but being average is not only the developers fault, it could also be the company's.