I think truthlover is probably right about the way the writer is using the story of Melchizedek. The early Christians (and many later ones as well) see the past as a type or shadow of the kingdom of God at times, and so the real person of Melchizedek speaks to a much greater truth, and in that sense he could be a human being, some suggest Shem, that ruled Jerusalem and was the priest there.
But there are other interpretations, even some that he was Christ Himself somehow, but I think that's probably wrong.
It is though very curious to refer to the order of priesthood of Melchezedek, and in one sense it is a bit of a slap in the face of the Levitical priesthood, stating that God has a higher order of priests he chooses sovreignly without the ordination of men, and I think that's a cool thing as it suggests that each individual Christian is uniquely and individually called of God.
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