Using terms like "ridiculous" and "dead cert" do not take away the fact that the quote is open to interpretation. If anything, the word "world" is what makes it ambiguous. When people talk about fictional universes, they usually use that word: universe. "World" can be used to mean environs, concerns, sphere of influence. I think unless more precise wording is used in a second quote, or something establishing the Vienna from The Memory Box is not the same one from The Shadow Heart, making such a drastic decision is premature.