Hmm. I would say that's a bizarre way to use the word "complete", but that doesn't prove they're not using it that way anyway. It's possible.
Why would it not make sense or seem 'bizarre'? The principles and agenda of Dungeon World are PART OF THE RULES, in fact a very important part! They bind the GM to saying fairly particular kinds of things when framing a scene or describing a gm move, etc. That being said, completeness in my book is more about a 'fully described process'. DW is complete in that it always describes what to do next. That 'what' may involve considerable judgment and leeway, but it is a qualifiable thing. IMHO D&D generally lacks this trait that what comes next is fully described. I think it sort of piecewise approaches it in the case of 5e, but it never quite gets there. TBH I think the main impediment here is the "GM-Centric" attitude of the designers of D&D. When AW was written, the designer went back to square one and thought about the process of playing the game. The 5e authors thought about how to inform a GM-as-game-purveyor as to the best way to do their job. 5e kind of 'backs into' the ideas of process of play, but it never quite puts them front and center in the way AW or DW (etc.) do.
The principles and agenda statements that pretty much every PbtA has are a part of this, clearly stating what the game is designed to be about and what to be guided by in the application of imagination and judgment when executing the 'game loop'. As with core game loop stuff itself, D&D first starts from the position that the GM doesn't have these constraints, and then tries to instill them in bits and parts throughout the rules, somewhat inconsistently, as pieces of advice. Its a lot less effective, IMHO, than the GM chapter of the DW rules starting on P159, which states them unequivocally and concisely. The opening of the chapter says it all:
"
How to GM
When you sit down at the table as a GM you do these things:
- Describe the situation
- Follow the rules
- Make moves
- Exploit your prep"
and then:
"Agenda
Your agenda makes up the things you aim to do at all times while
GMing a game of Dungeon World:
- Portray a fantastic world
- Fill the characters’ lives with adventure
- Play to find out what happens"
and finally:
"Principles
- Draw maps, leave blanks
- Address the characters, not the players
- Embrace the fantastic
- Make a move that follows
- Never speak the name of your move
- Give every monster life
- Name every person
- Ask questions and use the answers
- Be a fan of the characters
- Think dangerous
- Begin and end with the fiction
- Think offscreen, too"
Naturally each of these bullet points is further visited in the text.
The whole chapter is 25 pages, and includes the advice on how to run a fight, how to interact with NPCs, and several other similar things. It is, IMHO the most concise and effective presentation of GMing techniques in existence, at least in the context of the game and style of game it is presenting. No edition of D&D IMHO has anything approaching this.
And I want to state, I tie this all the way back to the OP and the question "what is the purpose of rules" and HERE IT IS. Between the core rules description of chapter 1 and this 25 page GM chapter we have a 50 page (and in 5e's format this would be more like 12 pages I'm guessing, as the typography is much less dense) we have the complete description of the process of play of a substantial RPG capable of portraying the entire range of play which might be found in D&D games, including everything both GM and players need, every rule they will have to follow and activity they will have to undertake in order to play (aside from system specific stuff like chargen) fully and unambiguously described. This is the purpose of rules, to define HOW TO PLAY, pure and simple! It includes how the GM and the dice will introduce 'unwanted fiction' which will form obstacles for the PCs, how and when each participant should have input, and the nature and form of that input, the constraints on it when it is open-ended, etc.
So, I would boil down "how to play" as being "what activities do the participants engage in which constitutes play" and "what are the constraints on these activities which limit the form of this play" and maybe the more general "what does it mean to enter into play" though I think that aspect is a bit less concrete in that it is effectively "submit to the rules and pick a role within the game" and applies roughly to all games in about the same way.