Without reading the sourcecode/proof (bookmarked for later) my guess is that it simply means something along the lines of "the user cannot execute code a la eval". So basically the user can do exactly the specified actions and nothing more.
More exactly, we check that the only calls when the user is not logged in are: ReadFile, ListPosts or Log: https://github.com/clarus/coq-chick-blog/blob/master/Spec.v#...