The twelve tribes were certainly a kingdom, but it was always a spiritual community entered through circumcision which is why all of Abraham's servants were included in the initial circumcision. The question is authorial intent and there is evidence that 12 tribes at the time of writing was being used in an end times sense regarding the promises of God.
I'm not saying this is absolutely the case, merely that an argument can and has been made in which the 12 tribes appelation is directed not simply at physical descendants of Abraham but to all the heirs of Abraham. Just as Paul in Romans states that not all Israel is Israel precisely because Israel has always been a spiritual reality rather than a strictly physical one.