There is not a big deal in this error from the author, but its interesting that somebody from Microsoft itself makes this mistake after its revealed in the disposal best practices.
Chapter 3: Introduction o the Sharepoint Object Model.
using(SPWeb rootWeb = site.RootWeb)
This is not a good coding practice because the RootWeb does not need disposal, I had SPRequest errors in my log in the past because of this exact same line. SO AVOID IT.
this is documented here: