If the post was inserted before the previously last post,

What the author was referring to was a conflict between the chronolgical order of posts and their appearance in the thread. The post to which he referred was last in the thread, by virtue of having been the last post received. Now, due to a problem with propagation from a certain server, it's still the last in the thread, but not the last received. It was "previously-last" but is not any more.