During the development of Stack Overflow, Joel and Jeff talked about the answers staying up-to-date in a wiki-like fashion. I think what the author means is that in order to participate in this - i.e. edit and improve an existing answer - you have to have a certain amount of rep.

It's quite likely that a user Googling for their particular issue might find an incomplete or out-of-date answer on Stack Overflow. When they finally solve it they are unable to improve the existing entry, which is unfortunate as for many "long tail" problems they are best placed to contribute.