Does Dungeon World allow a player, or obligate them, to 'author content outside their character'? I don't think it does. Possibly the handling of session 0 is an exception, as the initial establishing process that ends with the PCs being framed into 'Scene 0' includes things like establishing backstory, and clearly nobody is in character yet. Beyond that, Dungeon World 100% absolutely demands that fiction come first, and fiction come last, and that the fiction be addressed by and to the PCs, not the players. That is, if you play as intended, once the session begins everything the player says and does is in character. Even the selection of moves is in character, as the specific move being invoked is decided by the GM based on the fictional description narrated by the player, which SHOULD be delivered in character (though its pretty common for it to include some 3rd person language at times).
So, for example, if the party is tromping through the forest and the question of what is to the west comes up, the GM might say "Throndor, what lies to the west of the forest?" and the player, AS THRONDOR, answers with whatever knowledge Throndor possesses. Now, it is true that the player might have INVENTED some or all of that knowledge on the spot, but they are still IN CHARACTER and all they 'author' is Throndor's response to a question (and the GM could and should hopefully pose it through the medium of another character, though that isn't literally stated as a necessity).