There is no "the GM's role in PbtA games". The role of the MC in Apocalypse World is pretty clear from the rulebook, and it involves prepping threats and fronts (after the first session). There is no reference to prepping situations. Maybe you're thinking of Dogs in the Vineyard?
I honestly can't tell if you're being pedantic about phrasing here or my use of the word GM instead of MC. Maybe you're unaware of the fact that there are PbtA games out there that don't use the word MC. Maybe you forgot that you
quoted the line in question and aren't aware that "situation" means "a set of circumstances" and therefore the two words can be used interchangeably.
Also, MC = GM. Just like Keeper = GM and Storyteller = GM. Same purpose, same role. They use different terms for the sake of mood or to be different. The fact that you said that there is an MC's role means you're aware that there's a GM's role. The only difference is in what actions the GM can take in each game.
Maybe you can
stop being so nitpicky and actually deal with what I wrote instead pretending to be stymied by a marginally different word. If you don't want to deal with what I wrote, then don't reply at all; this obnoxious "you said
X when you really meant
X" is tiresome.
This question doesn't make any sense. It equates an event at the table (making a roll) with a state of affairs in the fiction (the existence of an exit).
Which is what happens. Player/character separation is not meant for you to pretend that what your PC does isn't determined by what the player says that the PC does. You like to say that the game is imaginary. That means it would have to be imagined by someone--the players.
Or to use your example: a player made a roll at the table; as a result, the runes meant what the players wanted it to/hoped it would mean in the fiction.
You may as well ask, of a group using the DMG Appendix A random dungeon generation, "Was the corridor/door/room/whatever there all the time, or did you have to roll for it?" Like, yes?
Pretty sure that the people who made those tables did so with the belief that the GM would be using them to make the dungeon
before the game started, not during play. So yes, the object was put there
before the players ever entered the dungeon. And even if they were meant to be used
during the game, they would still be rolled before, or at least
during the time that the PCs actually entered the area, so the door would still be there before the PC asked about it.
What
you're saying is that the player would roll to see if their hopes came true and there is a door, and if so, the GM would then have to keep rolling on the Random Dungeon Generator until they got a result that included a door. Or they roll to see if their hopes that the runes meant exit were true, they were, and the GM had to keep rolling on the Random Rune Translation Table until they got the result that meant exit. Which is obviously a rock-stupid way to go about things. Either the GM placed the door there ahead of time, or they didn't, the PC determined there was a door, and the GM
then placed it (without rolling on a random table). One or the other.