What is the thing that "non-believers already know" that these findings reinforce? I do not think that you will find a rational basis for any such thing in the facts cited by the article, as I shall discuss presently. This suggests that it is the tone and context of the article which provides your feeling of 'reinforcement', which in turn suggests that what is being appealed to is not your capability for reason, but your acquired prejudices.
The article presents a single scientific finding: the parchment on which the Birmingham Koran was written was very likely manufactured sometime between 568 AD and 632 AD. That is all. There is no other finding presented in the article.
The article presents four pieces of valid contextual information regarding dates. Firstly, that Muhammed most likely lived between 570 AD and 645 AD. Secondly, that Islam is generally regarded as having been founded in 610 AD. Thirdly, that the first Muslim community was established at Medina in 622 AD. Fourthly, that the first known 'official' version of the Koran was assembled on the orders of the third Caliph, Uthman, in 653 AD.
The article also clarifies it is known for a fact that many sections of the Koran had been written down well before 653 AD.
In this context, we have Keith Small's statement: "This gives more ground to what have been peripheral views of the Koran's genesis, like that Muhammad and his early followers used a text that was already in existence and shaped it to fit their own political and theological agenda, rather than Muhammad receiving a revelation from heaven,"
Clearly, the prior existence of the text of the Koran would establish a solid, factual doubt regarding the story of the Koran's origins. But just as clearly, no such thing is established here. The evidence presented merely fails to eliminate the possibility the Koran could have been written before so-and-so date. The parchment could have been written on before 610 AD, for instance. But even if it had not been, the possibility that parts of the Koran predated Islam would remain, because there is no way to disprove a hypothetical.
Thus, it is hard to see how the assertion that 'more ground is given' makes any sense outside of a prejudicial, opinionated context. There is no ground given that was not already there. What Keith Small is announcing is basically that the available evidence fails to disprove his admittedly fringe hypothesis. What he is overlooking is that the evidence we are discussing could only ever has (a) proven, or (b) failed to disprove his hypothesis. It was never at risk of falsification.
There is no rational basis to regard the "failure to disprove" as significant, and to treat it as such is evidence a prejudicial and emotive involvement in the subject matter.