The supercommentary on this passage by Joseph ben Eliezer Bonfils (Tsafenat Paneah, second half of 14th cent. A.D.) explains Ibn Ezra's comment as follows:
Moses could not possibly have employed the word 'then' ('az), for reason demands that the word would have been written at a time when the Canaanite was no longer in the land, and we know that the Canaanite departed only after the death of Moses when Joshua conquered it. Consequently, it would appear that Moses did not write this word here, but only Joshua or one of the other prophets wrote it.... Now since we have to believe in the words of tradition and in the words of prophecy, what difference does it make whether Moses or some other prophet wrote it since the words of all of them are truth and were received by prophecy?
(Nahum Sarna, Genesis [The JPS Torah Commentary; Philadelphia: Jewish Publication Society, 1989], p. 91; emphasis supplied by Roy Gane).
Bonfils basically agrees with the apostle Paul's understanding of the Bible's authorship: "All scripture is inspired by God" (2 Tim 3:16). But Thomas Jefferson had a different approach to the New Testament Gospels, which he expressed in a letter to John Adams dated October 13, 1813:
We must reduce our volume to the simple evangelists, select, even from them, the very words only of Jesus, paring off the amphibologisms into which they have been led, by forgetting often, or not understanding, what had fallen from him, by giving their own misconceptions as his dicta, and expressing unintelligibly for others what they had not understood themselves. There will be found remaining the most sublime and benevolent code of morals which has ever been offered to man. I have performed this operation for my own use, by cutting verse by verse out of the printed book, and arranging the matter which is evidently his, and which is as easily distinguishable as diamonds in a dunghill.
(Excerpts from the Correspondence of Thomas Jefferson).
When we read the Bible, can we distinguish between inspired, authoritative messages from God and other materials that were merely produced by human minds?