What you describe is the interpretation that does make sense. You are looking at properties of possible ways that the single "real world" could be. But if you don't look at this question specifically in the context of the real world (the single fact possibilities for whose properties you are considering), then Moscow as an abstract idea would have as much strength as Mordor, and "probability of Moscow" in Middle-earth would be comparatively pretty low.

(Probability then characterizes how properties fit into worlds, not how properties in themselves compare to each other, or how worlds compare to each other.)