If xf-addon:rebuild fails (for example due to a version ID difference) the error message is This add-on cannot be upgraded.. I'd guess something along the lines of This add-on cannot be rebuilt. might make more sense?
Very small issue that ended up throwing me earlier when I was trying to extend the thread datawriter.
In XenForo_DataWriter_Discussion, this:
* Data about the discussion's definition.
* @var XenForo_DiscussionMessage_Definition_Abstract