Whether or not the story is true, I think that it certainly illustrates how many Protestants approach the Deuterocanon (which they call “the Apocrypha”).John the Grammarian, a Coptic priest living in Alexandria at the time of the Arab conquest in 641 AD, came to know ‘Amr, the Muslim general who conquered the city. The men were each other’s intellectual peers, and John became the Emir’s trusted adviser. Soon, John grew bold enough to ask ‘Amr what might be done with the ‘books of wisdom’ held in the ‘royal treasuries’, going on to tell him of the great collections amassed by Ptolemy Philadelphus and his successors. ‘Amr replied that he could not decide the fate of the books without consulting the Caliph, Omar. The Caliph’s answer, quoted here from Alfred Butler’s Arab Conquest of Egypt (1902), is infamous: ‘Touching the books you mention, if what is written in them agrees with the Book of God, they are not required; if it disagrees, they are not desired. Destroy them therefore.’
The argument, in a nutshell, is that if the Deuterocanon either contains doctrines not otherwise found in Scripture, or it doesn't. If it does, it's heretical and erroneous. If it doesn't, it's irrelevant: redundant, merely edifying at best. Sometimes, this catch-22 is presented subtly. So, for example, from Edward C. Unmack's influential 1929 essay, Why We Reject The Apocrypha, he argues that on the one hand that:
So we can reject “the Apocrypha” because they merely illustrate the truths already found in the “Canonical Scriptures.” As the Caliph would say, if what is written in them agrees with the Book of God, they are not required.A further survey of the Books of the Apocrypha makes evident the fact that they are really supplementary in character to the Books of the Old Testament. [....] They really belong to a class of Jewish literature called the Haggada, in which historical, biblical, and allegorical types were employed to illustrate the text of the Canonical Scriptures.
But then Unmack criticizes “the Apocrypha” for teaching truths not found in the other Scriptures:
As the Caliph would say, if what is written in them disagrees with the Book of God, they are not desired. Destroy them therefore.Moreover, in the Apocrypha there occur unscriptural fables, fictions and doctrinal errors. Compare Tobit vi. 1-8; Judith ix. 10; 2 Macc. ii; Bel and Dragon, etc. Alms are represented as having power to earn merit. Compare prayers for the dead in 2 Macc. xii.
This is a simple, if stupid, game. Certainly, plenty of critical scholars have played this game with the New Testament: rejecting the authenticity of the Gospel of John because any new details must be concocted, while any old details must have been stolen from the Synoptics or Paul.
Of course, what Unmack, the Caliph, and the critical scholar overlook is a third option: that these other Books, be they the Deuterocanon or the Gospel of John, contain additional information that doesn't contradict other truths. Assume that you know that Tom lives in Kansas. If I then tell you, “Mary lives in Kansas,” I'm saying something that is neither redundant nor contradictory to what you already know.
So this whole line of argumentation against the Deuterocanon only works for Protestants if they start with the assumptions that (1) they have the full and complete canon of Scripture, and that (2) no theological truths may be found outside of these books. But those assumptions - about the truth of the 66-Book canons and sola Scriptura - are precisely what's in dispute.