For Atheism to even be valid, an imaginary God must exist in the minds of others, if that imaginary God didnot exist, then Atheism would be imaginary.
No. In order for atheism to be valid (ie, the belief that there is no god), there must simply be a belief that there is no god. The fact that the belief exists makes the belief a valid one. It might be a mistaken belief (in otherwords, there might actually be a god), but it is still a valid position to take.
Think of the flip side. If atheism is true (in otherwords, there is no god), then that simply means that theism is mistaken, not invalid, because there are people who believe there are one or more gods in existence. Since they hold this belief, it's valid, even if mistaken.
As far as the percentages, I suspect some of them overlap. The 10% Muslims he mentions are probably part of the initial 63%, with 26% being agnostic, and 10% being atheist. This makes 99%, with the remaining 1% probably saying "don't care" on the survey. In all likelihood, the Muslim percentage was seperated to say, "Even people who believe in the wrong god at least believe in something."
