I’d suggest the example is wrong and it should be
threaded, if you install Quip then the parameter list has
threaded not threading.
As to what it is then I suggest you look to the author’s blog, e.g.
http://splittingred.com/technology/2010/06/adding-a-twitter-feed-to-your-site.html#comments
Where you’ll see that some comments are indented from the one above. These are threaded comments where the indented comment is in response to the earlier comment, rather than just added to the list of comments.
Regards
Adrian Cherry