It seems to me that asking about the state of something in "your" Everett branch while it's outside your light cone is rather meaningless. The question doesn't really make sense. Someone with a detailed knowledge of physics in this situation can predict what an observer anywhere will observe. But in general, your point is correct. We do have a very hard time trying to learn about events outside our light cone, etc. But the message in the quote is simply the idea that an uncertain map != an uncertain territory.