Does the list of all articles include posts in discussion?

No.

If not, is there another list that does?

http://lesswrong.com/r/discussion/recentposts

Finally, is there some interesting reason that it stops in August 2012

It seems useless to me, but I sometimes still update the list when people bring it up (I've just updated it). The original purpose was to have a place to reference concepts that correspond to articles, but people are no longer doing that (for the most part, references to the concepts only got filled in for the earlier posts from the Sequences).