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It's quite likely that a user Googling for their particular issue might find an incomplete or out-of-date answer on Stack Overflow.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.
This used to be true, but there's a feature currently in testing that will allow anyone (even someone coming from Google) to propose an edit to a post. This edit proposal can be approved either by a moderator or several users with sufficient rep. Hopefully this will make Stack Overflow more wiki-like and help solve the problem with answers getting out of date.
This sounds similar to how Discogs has implemented quality ratings (via votes) for arbitrarily edited content, in between full editorial control and open wiki.
The temporal nature of best practices is my biggest complaint with how SO works. It works great for relatively static subjects, but its useless for fast moving technology targets due to the community resistance of duplicate questions.
It's quite likely that a user Googling for their particular issue might find an incomplete or out-of-date answer on Stack Overflow.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.