This historian needs to get out of his box and think about the ancient world. Cities did not exist on their own, they were either part of an empire, in league with an empire, operating under the protection of an empire, or in so many treaties that they cooperated with every empire known to them.
Rome was part of the Etruscan empire. Rome overthrew the Etruscan empire from within, turning it into the Roman Empire. Then they expanded it. Rome operated totally as an empire, they existed in that form, it was their mindset, their culture, their whole way of operating. The Senate didn't exist just to govern the city of Rome, it was governing the empire, much the same way our Senate thinks today... only more so!