I believe you are referring to the Sana'a manuscripts.
Carbon 14 dating indicates that some of it's manuscripts date from the second part of the 7th century(form about 645-690 CE) while Calligraphic dating points out to a few decades later(for several manuscripts).
The problem with this is, how do you know that it wasn't written by some of his followers? It is true that it appears a couple of decades after Muhammad's death but that doesn't prove that it was written by him.
Correlation doesn't imply causation, which is an important concept in science. The fact that the oldest surviving manuscripts of the Quran date back to the 650s, it doesn't prove that it was written by him.
As Carl Sagan said, extraordinary claims require extraordinary evidence. Muslims claim that the Quran is the direct word of Allah, written down by the prophet Muhammad. Well, such claim requires better evidence than an old manuscript from the second part of the 7th century.
In the end just like shakespeare, we will never know who exactly wrote them and this is were FAITH true FAITH comes in. Not just saying I believe but inside your core knowing that this belief( be it in Jesus for a christian or mohammed as the prophet for a muslim ) speaks to the essence of your being and calls your soul to rejoice in the Lord. The difference with atheists and theists is that we can look past our senses , atheists can not.
Charles Dickens plays this out well, with scrooge and marley
"You don't believe in me," observed the Ghost.
"I don't." said Scrooge.
"What evidence would you have of my reality, beyond that of your senses?"
"I don't know," said Scrooge.
"Why do you doubt your senses?"
"Because," said Scrooge, "a little thing affects them. A slight disorder of the stomach makes them cheats. You may be an undigested bit of beef, a blot of mustard, a crumb of cheese, a fragment of an underdone potato. There's more of gravy than of grave about you, whatever you are!"
Scrooge was not much in the habit of cracking jokes, nor did he feel, in his heart, by any means waggish then. The truth is, that he tried to be smart, as a means of distracting his own attention, and keeping down his terror; for the spectre's voice disturbed the very marrow in his bones.
To sit, staring at those fixed glazed eyes, in silence for a moment, would play, Scrooge felt, the very deuce with him. There was something very awful, too, in the spectre's being provided with an infernal atmosphere of its own. Scrooge could not feel it himself, but this was clearly the case; for though the Ghost sat perfectly motionless, its hair, and skirts, and tassels, were still agitated as by the hot vapour from an oven.
"You see this toothpick?" said Scrooge, returning quickly to the charge, for the reason just assigned; and wishing, though it were only for a second, to divert the vision's stony gaze from himself.