Yeah, these are really independent markets, in the same way that a store can be a monopoly even if it is only a monopoly "West of the Mississippi" with another

Yeah, these are really independent markets, in the same way that a store can be a monopoly even if it is only a monopoly "West of the Mississippi" with another not-actually-competing store in the East (which might or might not be a monopoly there; in the case of mobile app markets it is, and the result is really two monopolies, not one duopoly).