I too have played a lot of L4D. What I find interesting about L4D is the way the game changes dramatically as go up the difficulty levels. On normal difficulty

I too have played a lot of L4D. What I find interesting about L4D is the way the game changes dramatically as go up the difficulty levels. On normal difficulty you can just run'n'gun. On expert you'll be knocked down in five hits, so you need to be much more cautious (unless you have amazing reflexes). Manipulating your environment becomes much more important, particularly the use of throwable items redirect the zombies or stop them attacking from all directions. Tying this back to the article, the L4D bot AI, in my opinion, is intended to just be good enough to get you enough experience to play online reasonably competently. Multiplayer is the real game, and perhaps PvP is the real real game (I haven't played it much.) Bots are pretty poor at tactics, just bunching up by the player. They also don't use throwables at all.