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Even if we held onto that position (it's not the traditional position held by the exegetes). The gospel of Thomas was found in 1945 and it was also assumed to have existed without a manuscript in our hands. I don't see why it is so hard to expect that there are other lost manuscripts, see what Origen says
>You should know that not only four Gospels, but very many, were composed. The Gospels we have were chosen from among these gospels and passed on to the churches. We can know this from Luke’s own prologue, which begins this way: “Because many have tried to compose an account.” The words “have tried” imply an accusation against those who rushed into writing gospels without the grace of the Holy Spirit. Matthew, Mark, John, and Luke did not “try” to write; they wrote their Gospels when they were filled with the Holy Spirit
>The Church has four Gospels. Heretics have very many. One of them is entitled According to the Egyptians, another According to the Twelve Apostles. Basilides, too, dared to write a gospel and give it his own name. “Many have tried” to write, but only four Gospels have been approved. Our doctrines about the Person of our Lord and Savior should be drawn from these approved Gospels. I know one gospel called According to Thomas, and another According to Matthias. We have read many others, too, lest we appear ignorant of anything, because of those people who think they know something if they have examined these gospels - Hom. Luc. 1.1-2, trans. Joseph T. Lienhard
pic rel is another one that Origen predicts to exist, he was trustworthy enough to correctly tell us about that other gospel so we shouldn't discredit him and what the text says when it mentions that this is actually written down. And guess what that verse is cited in some form in the hadith https://sunnah.com/bukhari:7498 so it is not impossible for us to assume what was quoted there existed during 7th century Arabia