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Actually, it makes sense, the battle frontier wasn't implemented until emerald, and since these are remakes of ruby and saphire, they found a way to implement the battle frontier, but at the same time mantaining true to the originals, the battle maison is just a placeholder for when the frontier is built, whenever that is.
My point is that it's very possible for the frontier to appear in the game after a while, like the timed events in the demo.