Don't messages sent by you come back in a list together with any received ones?
Oh wow, you're right! Somehow I totally missed that. Even Thunder displays sent messages.
So you're right. You could get all incoming messages from a specific user by passing creator_id but you just couldn't get all the message you sent to a specific person without getting everything and filtering them.
Admittedly one of the benefits of Reddit was the extensive modding community that kept on top of things like this. In some news communities they would remove duplicate stories, even from different sources, for exactly the reasons you mentioned. Of course they could be overzealous too. :)