So the 1e PHB actually states that this ability is a magical summoning
But as you go on to quote, the DMG makes it clear that the PC receives a magical vision - of a horse that already exists in the gameworld.
the player isn't authoring anything... the rules of the game itself set the parameters for the "quest"
That's not in dispute, but a game like FATE sets parameters, too, for content that a player can introduce.
The point about authorship is that it is the player's choice, not the GM's, that makes this content part of the shared gameworld.
all the player does is decide when his character will undertake the quest.
No. By deciding to undertake the quest, the player also makes it true that there is a quest that is there to undertake.
Contrast the player of an AD&D thief: s/he can always declare that his/her PC goes off to find a mark to rob, but has not power to make it true, in the gameworld, that there are any marks having anything worth robbing.
Hence, the player of the thief lacks player authorship powers of the sort that the paladin player enjoys.
IMO, this is akin to claiming that by adventuring beyond the DM's created map the player is authoring new content because the DM now has to construct what lies beyond the border.
No. Apart from anything else, the GM can declare that the map marks the end of the world. Or that there is an impassable cliff, forest or ocean.
And even if the GM decides not to do any of these things, the players have no authority over the character of what the GM introduces. The paladin player does - there must be a horse nearby in circumstances of a level-appropriate challenge.
Based on the above, your definition of player authorship would then also include character creation when introduced, nevermind character backstory, since a character is "new content introduced into the shared fiction of the campaign world, which otherwise would not have been there". It makes it true that this character exists and was born n years ago.....etc
Making player characters is definitely an episode of player authorship. Hence all the debates, played out on these boards month in, month out, on the authority of the GM to restrict the sorts of PCs that players may build; to veto, alter or insist on various sorts of PC backstory, etc.
These are debates about who has, or ought to have, authority to author shared content.
It feels with such an open all-inclusive definition, the term loses much of its value.
I don't think so. First, there is no universal agreement on how PC generation should work, and what the relative weight of authority is enjoyed by players and GMs.
Second, if you move beyond PC generation to actually playing the game, there is as much if not more controversy over how authority should be distributed.
It's like claiming a spell authors setting content into the world.
Not really, because the paladin character doesn't magically make the horse appear, nor the evil fighter who is guarding it.
That said, the view that spells in D&D - or, at least, spells that give players a high degree of discretion to create new content (Wish would be the poster child here) - are player authorship plus a veneer of ingame explanation is widely held.