The reality of the game world exists even before the DM states it publicly. It must, or else the DM wouldn't know how to declare it. Whether that determination takes place immediately before the declaration (i.e. the DM hadn't thought about it, and needs to make a judgment call on the spot) or weeks/months/years before (when the DM is generating maps and notes for the future) is not terribly relevant.
Nope. I recently retooled a section of my world map that the PCs hadn't explored yet. Because I hadn't stated any facts to them about that area, nothing about that area was yet factual. I have not yet been arrested by the RPG Police, and my players have not yet complained about my abusive decision (because they haven't got a clue that I did it), so I'm pretty sure I'm in the clear.
The only important aspect of that determination is that it is made honestly, based on the DMs understanding of the world and how it works; and that it doesn't depend on meta-game factors, such as what the player might think is a "cool" idea right now.
Nobody does this ever. Not me. Not you. Not even David Eddings. What, do you think the Orb of Aldur exists because it just so happened to make sense for it to be created in this imaginary universe he was simulating in his head? Of course not! He imagined the universe in the first place because he thought it would be a "cool" idea to write a story about a magic rock (because he never met a cliché he didn't like -- but that's beside the point). And you and I and every other DM run D&D campaigns for our players because it's "cool" for them to be heroes and fight dragons and whatnot. If we're just depicting events based on an "honest" understanding of the world and how it works, 99% of the time, the PCs are peasants who spend an uneventful day tending to their fields. And the 1% of the time that something more significant happens, it's
still not a dragon, because dragons are unreal and, in fact, physically impossible animals, and the only justification for imagining them is that they're pretty damn cool. So if we're disallowing decisions about the game world being based on coolness, we have to assume that this significant event is instead, oh, let's say a dysentery outbreak. Constitution save to avoid pooping to death! What a compelling narrative we are weaving!
Meta-gaming is cheating, and explicitly against the rules of the game.
I guess we can add "explicitly" to the list of words that you do not know the meaning of. Either that, or you are deliberately misrepresenting what the rules
do say, even though we've had this conversation before and I've quoted them directly at you. But that would be disappointing. And perhaps... oh, there's another D word for it... it's right on the tip of my tongue...
...nope, can't bring it to mind. Oh well.