I think it is can be equally true if someone else authors that part of the fiction, but it depends a lot on how the fiction is authored.
@Enrahim had some nice posts about this a while back. From what I recall, the point is that the GM is constrained in authoring that part of the fiction by the fixed details of the world.
For example, in the rune case, if you go to investigate and the GM says "hmm, I think it would be nice for the players to get out of here. Let's make it a map", then you aren't exercising much agency as actor, because the GM's authorship overrides your decisions as actor.
But if the world had already been authored such that the runes are a map, then the GM has to present them as a map. Then you exercise a lot of agency as actor, because the direction of the fiction is deterministic with respect to your decisions.