The advice there is don't let the players ruin the game for themselves.
This sort of attitude was EXACTLY my motivation for the thing about "adults" in the OP. In both other sections of the DMG as well as other posts on this thread, people mention the philosophy that players and DMs are somehow co-creators of the so-called "story" in the campaign. But then pretty obviously, and quickly - both here and in the quoted section of the DMG - the DM suddenly becomes a tyrant.
If the narrativist gaming style is one that everyone is on board with, then why does it seem so natural for DMs playing in that style to lie to his players about it? The DM can say "hey, your scry fails because otherwise the adventure would be ruined." and the player will say "Wow, man, thanks for saving me from ruining the game for myself! You're the greatest DM ever!"
-if scrying will destroy a complete adventure or campaign, then I'll throw something in the way or otherwise show them something really misleading.
This raises the question of exactly what the definition of "destroy a complete adventure or campaign" is. Do your session notes burst into flame when the PCs kill the BBEG? The only thing this seems to me to be destroying are *your expectations* - and it's very presumptuous IMO, though consistent, to get those mixed up with the campaign.
So maybe it would help to be clear about exactly what role you expect players to play in determining the events of the campaign (which you probably have been with your own group, but IMO the PHB and DMG have not been). If you want DM to have "plot action points" that he can use to thwart player actions for no other reason than say so. In fact, since it's all so much good for the players then say so in a large disclaimer in the beginning of the Players Handbook. Then everyone is being honest about the game.
But...I've been DMing for over 30 years. If you're talking about a new DM, sometimes it doesn't hurt to say, "You can't plan for everything, don't let simple solutions solve complex problems." In math tutoring speech, consider the highest number problems on the SAT--if the answer is too obvious or easy, it's almost certainly wrong.
Well, I hope you're directing this advice to the new DM, because saying "no, it doesn't work because I don't want it to" is a pretty simple solution isn't it?
Secondly, I really don't think actual thinking people of the professions that use math or even logical thinking go out of their way to create complexity. In fact, people win medals for coming up with simple proofs and solutions to things that were previously complex. In my little subculture, it's actually a virtue to not be pedantic.
A solution to a problem doesn't stop being a solution if it's simple. The only reason to be second guessing the psychology of the SAT designer is that you really don't know what the answer is supposed to be. The real world IME is not multiple choice, so you're often stuck with having to know what you're talking about.