Guidebooks are not a part of the comparison you're trying to make, anyway. "Canon" means events that are defined as officially part of a linear story. Smash Bros is obviously non-canon, as it is not an official part of those franchises's defined stories.
Guidebooks are meant to provide insight on events in the storyline. They should be considered as "relevant" to a debate by default, unless someone with higher capacity explicitly states they're not to be trusted or take too much liberties with the original story, as is the case with Hyrule Encyclopedia.
Like I said, we need to stop treating fictional stories as if they take place in the real world. You can't go out of your way to ask a fictional character if they're universe or galaxy busters on-screen. The story follows an unbreakable, set pattern, be it a book, video-game, comic, or movie.
In fact, as I previously argued, the guidebook makes for a MUCH stronger case than anything that could have been stated in-game. Majora's Mask cast is 99% composed of peasants that have no real reason to know Majora is universal or even planetary. Omniscient databook files, however, have absolutely all the reason to know what happened in the game's background lore. The only gap in their knowledge arises from actual, solidified contradictions, of which there are none in this specific case.
Hell, if it had actually come from a random NPC, I would have rejected it.
