Not a quote from you, but an example of this principle not contradicted, written by someone else:
And another:
And here, from you:
All of these hinge on the same thing: "What the DM already knows". How does the DM already know that? This has been, repeatedly, used as a "this isn't and can't be railroading"--but HOW "what the DM already knows" is left almost totally unstated. Like I've gone back through and seen multiple posts which reference this, even reference that the "how" matters...and then never actually say a word about the "how".
You may not have been the one to explicitly use the word "it's not a railroad", but the arguments are all there and they're all fundamentally the same. This is being given as a defense against railroading, but it does nothing of the kind. It simply shifts the place where railroading may occur away from "DM response to player input" to "how the DM decides what she already knows".
Here's how I approach it:
When I prep a sandbox campaign, I start by building a setting that would make sense even without the players. I populate it with factions, locations, conflicts, and situations that are plausible given the premises of the setting, not constructed around the assumption that players will interact with any specific element. This principle is crucial to avoiding railroading.
This material forms part of my "Bag of Stuff": a body of prepared details that give players meaningful choices once play begins. But none of it demands players act a certain way or follow a predetermined path.
Consistency with setting facts does not mean outcomes are preordained. Players are free to interact with, ignore, oppose, or support any part of the world, and because the world responds plausibly, the course of the campaign is genuinely emergent. Player decisions, not my prep, drive what matters and what changes.
Naturally, any setting reflects some aesthetic or thematic choices. But I work carefully to ensure that these choices establish the world's tone and initial context, not dictate the players' story outcomes.
One part of my approach that often gets overlooked is how this all manifests at the table: through first-person roleplaying. I don't just describe the world abstractly. The world comes alive when I roleplay NPCs in the first person, and players respond likewise.
This matters in two ways to the points you raise:
1) It grounds later discussion with players. If a question arises, we can review how I roleplayed the NPC based on the established notes and setting facts, for example, whether my portrayal of Thranduil in the Rhovanion sourcebook for AiME was consistent with the material.
2) First-person roleplaying ensures players are presented with environment where they have the full range of options that exist in real interaction. This makes it virtually impossible for me to control player choices or railroad them into specific outcomes.
This process is reinforced by my emphasis on referee impartiality. The referee's job is not to shepherd players to a "correct" outcome, but to present a living world, adjudicate fairly, and let consequences unfold naturally, even when it surprises me.
What the DM already knows matters. But the key is that the referee must build a world that invites free interaction, not one that hides a script the players are expected to follow. That’s the heart of running a sandbox campaign fairly.
Wrapping this up, this crucial topic is why my first book I wrote on the topic of Sandbox Campaigns, is about
How to Make a Fantasy Sandbox