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".