I guess this is not exactly a claim to "prove a negative" but it seems fairly close. And is therefore little more than mental masturbation. There's no point to the exercise, but it makes Dawkins feel good.
This statement means nothing. It is an expression of belief, not a proof, or even statement, of fact. So Dawkins' work *denies* the existence of God. So what? There are also works that deny the existence of the Holocaust, or the moon landing, or Elvis' death. Each would claim to be "thoroughly researched and expertly executed", and each would be essentially nothing more than nonsense supporting a nonsense proposition which is itself based on nothing but assertion.