Yeah, it's a TAS--
From what I gathered, there are certain events in the game that aren't actually triggered by the completion of dungeons, but rather the items you obtain (which in-game would be a result of completing a dungeon). With him going to get the gold scale, then the bottle with Ruto's letter, it allowed certain events to open up that otherwise would be unaccessable.
Even further, jumping through the Temple of Time's sealed door and becoming an adult, and grabbing some necessary items also helped trigger the ending. It's pretty sweet to watch D: