Oh ok. I don't really think all theories should be dismissed, in fact if a headcanon theory is made well enough and a vast majority of fans like it than it could maybe catch Nintendo's eye and slip it into already established canon. Of course the odds are slim that could happen but I sometimes...
Headcanon is generally accepted by a fan, or fans, but is completely fictional. Canon means the events follow what is generally accepted and established by the series by the developers. That is the difference.