Well... I certainly would not argue with people's assertions about the character of their own play. I would point out that, as a general concept, its hard to give that a huge consideration. That is, this sort of unrevealed backstory is pretty tenuous. One can always reason some way to get to X. Or if X really is very explicitly ruled out (IE this bad guy has an absolute aversion to lying, so he won't deceive the PCs) then there's always Y, which can be functionally equivalent to X, but avoids the restriction. I agree that such things can add flavor to the game, they're good, but I'm vastly less certain they constitute a substantive barrier to GM story telling. Anyway, beyond that, if they are derived entirely from the mind of the GM in the first place, its more a matter of temporal sequence of when backstory was invented, yesterday or today. I am not sure that's a big distinction.
And, in general, contingencies are rarely going to hinge on a specific trait or resource constraint, etc. of one person or group. The world is complex, contains many factions and operatives, and whatever. With total backstory authority, setting authority, etc. you can generally bring about whatever eventuality you desire, assuming the players don't outright dig in and fight you to the death on it! Even then, the GM in some sense prevails, though obviously we're getting into degenerate cases at that point.
Again, this is why dungeons WORKED in the simple GM-directed paradigm of Gygaxian play, walls and such don't have loopholes or ambiguity. You either can or cannot proceed west, it isn't really a matter of interpretation, even if it was decided at some point by the GM; he's now just as constrained as the PCs.