That is, according to this post on the official Left 4 Dead blog.
As stated in the blog post:
The upcoming DLC is our experiment in not having a story tied to the DLC. While we are working on that, we also wanted to keep expanding the story and world of Left 4 Dead.
The Passing was decently good, especially given that it was free as all of Valve’s DLC is, but hopefully this will shape up to be better. Check back for more news about the Left 4 Dead 2 DLC as it is released by Valve.