1. You can’t just selectively quote parts of the text to support your point of view.
I mean … you can, but it will probably not be looked on favorably.
2. There is something unnerving about being told the DMG contains no advice, but then told it contains bad advice, while realizing the DMG contains lots of good advice.
3. Finally, what you call waffling and contradictory is what other people would call attempting to support multiple common approaches. I feel,like this has been covered ….
We're really going to do this? OK.
The first bit I quoted, from the start of the book (which I note you did not feel a need to nit-pick) looks an awful lot like what one of my old professors would have called a 'programmatic statement.' That is, it's laying out the broader outline of what we should expect in the book. Here's another bit, right from the beginning:
The Dungeon Master (DM) is the creative force behind a D&D game. The DM creates a world for the other players to explore, and also creates and runs Adventures that drive the story.
And then:
Every DM is the creator of his or her own campaign world. Whether you invent a world, adapt a world from a favorite movie or novel, or use a published setting for the D&D game, you make that world your own over the course of a campaign.
And then the previously quoted:
Whether you write your own Adventures or use published ones, expect to invest preparation time beyond the hours you spend at the gaming table. You’ll need to carve out some free time to exercise your creativity as you invent compelling plots, create new NPCs, craft encounters, and think of clever ways to foreshadow story events yet to come.
This is a clear statement that the job of the DM is to create the world and steer the plot, as the main storyteller.
Then, we get all the caveats about letting players' actions matter, and, yes, a whole menu of ways to roll, including outright lying and cheating (whose downside amounts to 'the players might see through the illusion'!). I don't want to say 'impossible thing before breakfast', but it seems you've made me do it.
Within this context, the caveats and alternate approaches are
(1) bad, vague, and contradictory, and
(2) not truly supportive of multiple styles of play.
All of this is, once again, before we get to the lack of
actual procedures other than fiat for doing pretty much anything at all.