It turns out I violated my own rule for always looking at the source if possible, because it is clear that they cherry-picked their quotes, and other quotes in the same chapters show their interpretation is wrong.
[T]he promise looks very different if we take seriously all of the offspring of Abraham. Genesis 15:4-5 has God taking Abram outside and telling him that his descendants will be as numerous as the stars of the heavens. Genesis 17:4, probably the pivotal text, has God saying to Abraham: “This is my covenant with you: You shall be the ancestor of a multitude of nations.” Many nations, a multitude of nations; many offspring, many kings—read Genesis 17 again and see the plural nouns here.But Genesis 15 says:
Close readers of Scripture will know that in fact Abraham did become the father of many nations. With Sarah he became the father of Isaac and the ancestor of all in his line, via Jacob and Esau. With Hagar he became the father of Ishmael and all in his line. And with the long-forgotten Keturah (Gen. 25:1) he became the father of Zimran, Jokshan, Medan, Midian, Ishbak, and Shuah. The Old Testament clearly positions Abraham as the father/ancestor of not only the Jewish people but of a vast number of other peoples, all scattered through the territories promised in Genesis 15. Abraham becomes the father of dozens of peoples, exactly as the Bible says! It is certainly true that the Old Testament primarily tells the story of the line of Isaac and therefore of what became the Jewish people, but that cannot cancel the significance of the promises to Abraham and the many peoples credited to him in Genesis.
And He said unto Abram: 'Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years; and also that nation, whom they shall serve, will I judge; and afterward shall they come out with great substance. ...And in the fourth generation they shall come back hither; for the iniquity of the Amorite is not yet full.'This is an obvious reference to the children of Israel in Egypt, using the same phrase "thy seed" that the authors refer to.
Likewise, Genesis 17 proves that the covenant goes only to Isaac, not Ishmael:
And God said: 'Nay, but Sarah thy wife shall bear thee a son; and thou shalt call his name Isaac; and I will establish My covenant with him for an everlasting covenant for his seed after him. And as for Ishmael, I have heard thee; behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly; twelve princes shall he beget, and I will make him a great nation. But My covenant will I establish with Isaac, whom Sarah shall bear unto thee at this set time in the next year.'
Later, Isaac makes it clear that the covenant is passing only to Jacob, not Esau (Gen 28, today's Torah reading:)
And God Almighty bless thee, and make thee fruitful, and multiply thee, that thou mayest be a congregation of peoples; and give thee the blessing of Abraham, to thee, and to thy seed with thee; that thou mayest inherit the land of thy sojournings, which God gave unto Abraham.'
I brought most of these to their attention in their blog, after their follow-up post where they said how "We are sincere in raising these biblical questions and hoping for a sincere answer. We are deeply serious about biblical authority."
But they never responded, not even on their own blog.
Now, I do not know if Christian Zionists are advocating that Israel conquer all land between the Nile and Euphrates; as the authors seem to imply. The boundaries of the land God is promising the Israelites seems to change in different chapters and I imagine that this is an issue that needs to be grappled with from a Christian perspective. But to facilely declare that God must have been talking about Arabs and dismiss everything else in those same chapters seems more an exercise in wishful thinking than in serious Biblical interpretation. I also do not believe that Israeli policy is made in response to Christian Zionist demands.
I may be completely wrong, as I am not a Christian and maybe there is a glaringly obvious error I am making in the literal interpretation of these verses. But if not, then based on their argument and the lack of adequate answers, the state of Christian Biblical scholarship has gone way downhill since the 19th century.