Hey, even in the Sigil blog (
here) Calibre is pointed as Sigil's successor. (In fact, I do not use Sigil anymore).
Don't you think this is one of the more prominent features of Calibre nowadays?
I really think you should update
http://calibre-ebook.com/about with explicit info about the capabilities of Calibre as a epub/azw3 editor.
Don't be SHY!!! Calibre is just incredible and the Editor has made it more incredible (if possible).