Thread:KingCannon/@comment-7218858-20170706023832/@comment-25501246-20170706184736

Horikoshi would have to state it is canon, either by inserting that stuff into his manga or by directly stating it. Otherwise, it's just borderline canon, at best.

Let's take an example from One Piece. The movie Strong World was directed and written by the manga author, Eichiro Oda. The movie even got a special manga chapter. However, only the special chapter is considered canon (because it's in the manga) whereas the movie is not, as the movie contradicts the timeline of his own story in the manga (while the special manga chapter doesn't). Oda also designed a few anime filler and movie villains, which doesn't make them canon either.

Horikoshi himself has made a nod to Trigger, but has not actually used its name in the story, meaning he doesn't want the continuities to mix up too much. The reason this happens is because Horikoshi (as well as many authors) do not want fans looking for several pieces of media just to keep up with a story (an issue Star Wars is very guilty of). After all, we do have two novels, an OVA and a few games where Horikoshi himself has led a hand to, but you'd have to be hard-pressed to call those canon, even though the fact the author worked on those also means that they too have their blessing.