This is a really bad argument.
The age of the animal skin that the document was written on does not tell us the age of the document.
It tells us the age of the animal skin...
It may just be that they erased earlier writings on the old animal skin and written a Quran on it...
Not only that, I don't see how historians can based on the age of the animal skin! It says:
"It is believed that the Birmingham Koran was produced between 568AD and 645AD, while the dates usually given for prophet Muhammad are between 570AD and 632AD.". Carbon dating is not 100%, there is always a range of dates and they presented it. It doesn't appear that the range of dates they presented really predate the years Prophet Muhammad is
believed to have lived. It may just be that the Birgminham Koran existed near 645 AD...