Any backstory that isn't contradicted by the films is considered canon. This is the rule at LFL. You don't have to agree with it, and are welcome to believe as you wish, but you aren't going to change that fact at all. Just because something isn't in the films doesn't mean it can't be considered to have happened in the films. I recall many times that threads would pop up in TFn about common knowledge "facts" about the SW films that weren't in the films themselves. For example, the "imperial walkers" in ESB are actually AT-ATs.