With respect, I don't think anyone is confused here, any more than they are confused when they talk about killing players instead of killing PCs.
You may find the language choice to be inaccurate, or that it conceals some other point that's valuable to make, but please don't equate casual language use to confusion.
Ok, with respect, since that went zooming by you without pause, let me be more blunt and say I think you are confused and that your rhetorical question indicates you fundamentally did not understand the OP's question.
Simply put, the rhetorical device "would you write the world, and the associated stories of how it progresses around and with the NPCs, if you didn't have PCs playing in the world?" only further confuses the question, "Does the world exist for the NPC's", because the answer to it is irrelevant and unilluminating regarding the underlying issue. The original poster did not ask whether the world exists for the players, so asking a rhetorical question about whether the work done is only done because the game is played doesn't address the topic - and that's leaving aside the fact that different GMs could reasonably answer "Yes" or "No".
Backing up, the question, "Does the world exist for the PCs?" or "Does the world exist for the NPCs?" doesn't have an obvious meaning, which I think leads to the confusion of people trying to answer the easier question, "Does the world exist for the players?", which is what you are apparently trying to answer. But the original poster in the other thread foresaw this potential for confusion and so defined the question by example, stating, "I am probably not being as clear as I'd like, so I will use an example: when you create a settlement, do you develop it with the PCs in mind (including making sure there's a shop for adventuring equipment, and some NPCs with adventure inspiring plot hooks) or do you develop it independent of the PCs with an eye toward whatever definition of realism or verisimilitude works for your world? This question could easily extend to other locations, to NPCs and to social structures."
Now, the answer to this question is mostly independent of the question, "Does the world exist for the players?", in that you could answer "Yes" to the question, "Does the world exist for the players?", and yet adopt either approach described by the OP, and as the OP noted you could in fact have some sort of mixed and complex approach. Essentially what the OP is asking about is GMing priorities. In your setting, does realism or simulation as a goal of play trump or usually trump other potential goals of play? For example, in some cRPGs the world is divided into zones according to the level of the PC's expected to explore them and a shop keeper exists that sells equipment appropriate to the zone - even if the shop keeper is in a hovel in the wilderness. Different GM's can approach this problem differently, deciding to forego simplicity of gameplay in favor of what strikes them as greater verisimilitude, or possibly starting with the desired structure and working backwards on how to justify it, or simply ignoring verisimilitude in favor of ease of play or perceived narrative benefit.
So talking about whether "would you write the world, and the associated stories of how it progresses around and with the NPCs, if you didn't have PCs playing in the world" doesn't address the OP's question who asked about "whether the world exists for the PCs" and as best as I can tell, doesn't address this poster's question either.
Unfortunately, the OP hasn't clarified what the seemingly odd question means in this thread, but he did give us a hint in the prior thread when he asserted: "I think a better question would be "Does the world exist for the NPC's?", considering the NPC's only exist for the benefit of the PC's, and they only exist when the characters are around."
So the question, "Does the world only exist for the NPCs?" seems to mean something like:
"Do NPC's only exist when the PC's are around?"
OR
"Are NPCs tailored according to the PCs needs (love interest, foil, appropriately level enemy, mentor, shopkeeper, sidekick) or do they start out as independent fully fleshed characters in their own right, living in a world which exists independent of the PC's and having needs and goals that initially do not revolve around the PC's?"
OR
"Do NPCs conform to some demographic standard which meets some standard of realism, or do NPC's exist only because the PC's story needs an NPC?"
Or some sort of question of that sort, none of which is definitively answered by the question, "Do the NPC's exist for the players?", because presumably, however the GM approached world building and the above questions, he could claim reasonably that he's doing the work for the enjoyment of the players.