Well, that's a highly contentious claim. It's not one that I accept. It's certainly not universal, nor even especially widespread, among RPG designers. It wasn't accepted by Gygax, who was one of the inventors of the game-form (he tended to accept the impartiality idea, but not the "objective description" one - see eg his discussions of hp, saving throws, XP and combat resolution in his DMG).
As tends to be the case, the RPG form wasn't perfected - or even really understood - until a while after its nominal invention. The thing which Gygax called an RPG does not qualify as such by the more-rigorous standards of the late eighties, although it would certain fit into a broader category of game that also include such outliers as FATE and D&D 4E.
If you want to say that all of those games are RPGs, and actual role-playing-based games are a different subcategory within that, then that's certainly an argument that you could make, though it sidesteps the relevant point.
I don't understand what constraint you think the last sentence imposes.
The important thing is that the integrity of the world comes first, and what you do with it is a secondary consideration. If there's a dragon because
it makes sense for there to be a dragon, then that's fine; if there's a dragon because
it would be cool if the party fought a dragon, then that's shenanigans.
I assume you're familiar with the practice of a GM asking a player, "Are you sure that's what your
character would do?" It's probably not something that you do in your own games, but it's common advice in Palladium games, and has appeared in some media depicting the hobby. Setting aside how the question can be abused by bad GMs, the point of asking is to get players to really
think about their characters, and to encourage them to role-play honestly (instead of being side-tracked by other concerns).
Well, the same thing can be used as a tool to help a GM better role-play their setting. Whenever there's doubt about whether to include a setting element (a friendly priest in town, a wooden crate, the Tarrasque, etc), the GM should stop and ask themself
"Am I sure that this thing should really be there?" And if they can honestly answer yes, then it's safe to include; but if they can't honestly answer yes, then including it anyway would be a violation of their obligations to impartiality.
If I think it would be fun for the PCs to meet the tarrasque (and I do) then I can trivially come up with a reason why they might
This is rationalization. It is a logical fallacy aimed toward disguising your true intentions (to other players, or to yourself). If your
real reason for including an encounter is that it would be interesting for the players, then it doesn't matter how cleverly you go back and make it fit, because it already didn't
originate from your honest presentation of the world.