Can you give specific examples of the things you're citing? Also how would the players know if something is authored at the table vs. beforehand and just presented at the table as if it were?
Yeah, sure. They are oft cited, but here goes (for Dungeon World):
The most basic facts of how to play are established starting on p17, where it is explained that the game is a conversation, and that the GM first describes a scene, establishing the current fiction, and then dialog determines how the situation evolves from there, and that moves may be triggered if the situation matches their trigger conditions. We've also learned that the game is about fantastic adventures, heroes, and exploration (starting on p7).
First of all is the fact of being No Myth. The GM cannot really have some big plans of their own if they don't know much more than the players and are making things up on the spur of the moment. This is reinforced by the Agenda item "play to find out what happens" which says "So really, don't plan too hard." (p161). This page also notes that you are "working with the players to create a world that is engaging and dynamic." This, and the sections about creating fronts make clear that the world is a collaborative enterprise.
The GM's principles are listed on p162. "Make a move that follows" is a big one, your GM moves must follow from the existing fiction. "Draw maps, leave blanks." again telling us we are not a solo world designer. "Ask questions, use the answers." LITERALLY ask the players lots of questions. The examples of play ALL demonstrate this frequently, and make clear that the types of questions are not just "what do you do?" but also asking for contributions of fiction and world building. "Begin and end with the fiction" reinforces the making moves that follow idea, when you speak, you start from the fiction and end with it, thus the GM is bound to honor the fiction, his and the players.
"Your principles are your guides. Often, when it is time to make a move, you'll already have an idea of what makes sense. Consider it in light of your principles and go with it, if it fits."
Honestly though the section from p159 to p177 is just suffused with the idea of working with what is immediately in front of you, that everything in the situation is provisional, at most partially planned, and filled with potential that has not been mapped out. p170, the dungeon moves epitomize this, even if a room or hallway is on the map, when the PCs enter it you can make a move like "Present riches at a price." This is not some key where a treasure is placed, its "Oh, you entered a hallway, OK, you see some gold pieces on the floor, but the whole area smells really weird, and the walls are stained green..." or "You run into 3 orcs who seem to be coming from the opposite direction!"
p177-185 describes the first session of play: This is fairly important, as it thoroughly establishes the No Myth nature of DW, the very first thing that happens in a game/campaign is character creation, at the table, blank slate. The GM should ask questions, and answer them as well, or turn them around and let the players do so. The GM should not be a afraid to say, "I don't know." Work together to find a fantastic and interesting answer.
The GM is told that they're RESPONSIBLE for the world, and have 'a lot to say' in what lives in it. "You don't need pre-approval for everything, but making sure everyone is excited about the broad strokes of the world is a great start."
"Once everyone has their characters created you can take a deep
breath. Look back over the questions you’ve asked and answered so
far. You should have some notes that will point you towards what
the game might look like. Look at what the players have brought to
the table. Look to the ideas that’ve been stewing away in your head.
It’s time for the adventure to begin!"
The game then starts in media res, generally. Starting with a situation stemming from the player's questions and answers, the GM describes the first scene of play, usually with some PCs being in a spot of trouble.
Use what they give you,
Establish details, describe
Ask questions,
Leave blanks,
Look for interesting facts,
etc.
After this first session the GM can create fronts. "Fronts are secret tomes of GM knowledge." These are the things that will happen in the world even if the PCs are not paying attention. They're a stockpile of moves for the GM to make whenever something is needed and the players didn't supply you with some direction to go in or an immediate response to them. One of the front's dangers can show up (see above where I mentioned dungeon moves). etc. There's a lot more on fronts of course.
The whole tone is repeated admonishments to collaborate with players, to leave things open, not to define things completely, etc.
The structure of PCs is also important. They have a race and class, but in DW the character's class is a unique trait, you are THE Wizard, not just one of the wizards, you are the dude! Characters have bonds, which state a relationship to another PC, and which you can get XP for fulfilling. Bonds belong totally to the players, they decide if they are resolved for XP, and get to write any new ones they like.
Finally every character has an alignment, and an alignment move to go with it. These moves are more like beliefs or instincts than anything else, but they belong to the character and its up to the player to decide what his character's alignment is and what move goes with it. However since these ARE moves, the GM is responsible for noting that they have been triggered. Each instance is good for an XP point.
So, nowhere in DW is it stated that GMs have NO creative agenda of their own, or no power to shape the game. Its just that a game starts as a blank slate, with the only things defined being the PCs, their race, class, alignment, and bonds. Everything else is coming from questions asked and answered, usually by the players, supplemented by the GM's initial scene framing, front planning, steading creation, etc. So, actually GMs have plenty of POWER, at least as much as in most other games. However, they are told to use it mostly to bring the world to life around the PCs and in respect of what the PCs are.
Honestly, after reading Apocalypse World 2e I can see why some people say that DW actually doesn't say all this stuff very forcefully by comparison. This post is long already, though, but if you want to read a game that is EXTREMELY clear about the non-existence of any GM agenda, read AW 2e!