The likelihood that the entire Koran was written from scratch after the birth (and during the life) of Muhammad is pretty slim. The Hebrew Bible was canonized* after the Babylonian exile and there have been copies found containing different versions of other books. The Old Testament has additional books to the Hebrew Bibles (Maccabees) that were never canonized in the Hebrew Bible but remain an Historical Jewish cannon (The Great Revolt). There are also varied versions of the New Testament such as additional Gospels and Epistles that were not canonized at all or did not survive transition from proto-Christianity into Orthodox Christianity (Or sub sects like the Coptic and Syriac Bibles).

The original works that make up the New Testament was most likely been written in Classical Hebrew, Hellenic Hebrew and Greek (Since some canonization was conducted in as late as 300-400CE also possibly Talmudic Hebrew), there were quite a few sects during the Hellenic period of Judaism that either converged into Christianity or into Judaism so there was quite a bit of cross pollination between various religious texts that some were canonized in the New Testament, some were canonized as the external canon in Judaism and some were lost for ever.

So it would be quite surprising if the Koran was a body of work that was exclusively written during the life time of Muhammad since it's much more likely that it's been gathered from existing works and rewritten and adjusted just like the Hebrew Bible to fit the required goal during it's final canonization. It is also highly likely that adjustments would've been made after the canonization it self since variations can be found both the Hebrew Bible and the Old and New Testaments, as far as the Hebrew Bible goes with the exception of the Hebrew Bibles that is used by surviving Judaism derivatives like The Samaritans, Karaim, and Beta Israel is standardized since the 9th century (a handful more recent versions were discovered with alterations over the years but not in masses) and the New and Old Testaments were altered with each new translation pretty much upto the modern era.

On a side note, I am for one actually surprised that this work was conducted dating early Islamic artifacts and texts tend to generate quite a big shit storm.

*I refer to Canon as the agreed on final collection of works that made it into each book, minor variances such as ones regional differences between existing schools and different translations are not treated as separate canon.