I believe they are using the term in the sense of canonical name (CNAME) which is an alias. As it applies to WP/WPMU, I think they are using canonical to refer to the rewrite system to provide a standard set of aliases (permalinks) that map to the actual content (ex. ?p=307).

My take of the question is, "How do you feel about extending/adding to the rewrite rules/system to cover additional media types and/or adding them to the core?"