As someone who runs a regular DW game, I agree--and I think practical demonstration is in order.
I've run a weekly Arabian Nights flavored Dungeon World game for about three years. (It still feels weird that it's been that long.) We tend to be not super strict about following the "correct" play of DW--that is, players sometimes think in terms of moves rather than in terms of fiction, and I don't admonish them for doing so--but by and large we try to keep focused on what's happening, the "...what do you do?" question, etc.
As GM, I am not at absolute liberty to establish whatever story I want. I have an awful lot of leeway for anything that hasn't been touched on yet, of which there is plenty. But there are several moves, both generic and specific, that constrain what I'm allowed to do, and there are under-the-hood things that constrain my behavior further. These things are not the Agendas or Principles that the book defines for me; these are actual rules which set the terms of my behavior as a GM. I'll give three examples: the Discern Realities move (effectively Perception rolls from D&D), the Bardic Lore move from the Bard class, and the distinction between Soft and Hard moves and when I'm allowed to use each in response to player actions.
Discern Realities is what the characters are doing when they make a focused, discrete effort to obtain more information about the world around them. As DW says, "you have to do it to do it": they shouldn't just declare "I'm discerning realities," instead the player should say, "I see the dresser. Does it have any disturbed dust on it? Are there any drawer handles that look shiny? How about the floor, does it look scraped like something's been moved a lot?" or something similar. Such questions mean the character is actually examining the world, and that triggers the following move.
As part of the GM side of these rules, I must answer these questions truthfully. I'm not allowed to lie--unless the roll is a miss, but I don't lie then either, I do something else (which I'll explain later). Now, just because I can't speak untruthful things, doesn't mean I can't leave out information if the characters wouldn't have any reason to perceive it, so I can still maintain some mysteries. But my freedom to act as a storyteller is expressly limited by these questions, and the rules expressly direct me to invent answers, where necessary, to make the result of partial (7-9) or full (10+) success interesting. That is a practical application of the Principles and Agendas, but not actually either of those things directly.
Now, compare this to the Bardic Lore class move:
Note, again, that I am constrained to answer truthfully, but this time it can be ANY question. Of course, the player is also constrained in two ways, the first being the nature of the entity (it must be in their area of expertise) and that it be their first time encountering it, the second that I, as GM, am then empowered to ask where the Bard learned this from, and the player must thus answer me truthfully. Thus, even though both of us are "inventing story," neither of us does so with free rein. We are constrained by what the rules permit us to do. This then leads to the possibility of, for example, developing the skill of "asking good questions," and the skill of "improvising explanations." The rules shape what we're allowed to make up stories about--both the players and the GM.
Finally, the distinction between "hard" and "soft" moves. To start, the GM making a move is structured in the "move" format:
A "hard" move is one with immediate consequences; a "soft" move is one without immediate consequences. So, for example, dealing damage to a character is definitionally a hard move, because that's a direct effect that will require effort or resources to overcome. The soft-move equivalent of dealing damage is to threaten a character with something: a trap springs, a monster swipes, the Duke stands to speak, etc. There are a variety of GM move concepts, such as "reveal an unwelcome truth" or "offer an opportunity, with or without cost." Notably, as GM, I am not allowed to simply inflict hard moves whenever I like--there must always be some trigger in the fiction for them, which generally (read: almost always) means the players either give me a golden opportunity (e.g. by ignoring a soft move I've already made) or, more commonly, by rolling poorly. I can, however, choose to use a soft move instead of a hard one if that makes more sense. My ability to weave a fiction--to tell a story--is heavily restricted by NOT being allowed to fling out whatever consequences I like.
As a practical example of a hard move, I offer my solution to the "problem" of missed Discern Realities rolls. See...Discern Realities doesn't give you a good notion of what "failure" should mean, particularly since Dungeon World is fail-forward in its philosophy. The first DW game I played in, another player (gently) exploited this loophole for some (effectively) free XP. And I couldn't blame him, but it did make me wonder what I should do to forestall this while dodging the stereotypical problems of Perception-type rolls. And then it hit me...I just needed to make a hard version of the "reveal an unwelcome truth" move. So, whenever the party rolls badly on Discern Realities, I tell them they MUST ask me one of the questions....but they'll get an answer they won't like. It will be completely true, but it will reveal that the problem is more dire than they expected, or that something troubling is going on, etc. This is generally a hard move because its consequences are immediate (they learn a true but bad thing right now), but it could be a soft move if the party can still forestall the problem (e.g. "the cultists are almost finished with their summoning ritual--you're almost out of time!")
Within this rules framework, our goal is still always to generate "the fiction" to our enjoyment--to tell a story. But that story is mediated through a set of rules that, in some cases, rigidly determine what things the players are allowed to do, and what things I as DM am allowed to do. Sometimes, I'm forbidden to do things (like making hard moves when the players haven't missed a roll nor given me a golden opportunity), sometimes I'm required to do things (like truthfully answer questions, or give a piece of information that is interesting and useful, etc.) Because there are these few but bright lines, it seems to me that there can quite easily be a sense of "skilled play," of leveraging the things you can (and can't!) do to achieve certain mechanical ends, even though those ends always and intentionally flow back into the story-telling experience that is the focus of play.
