Obviously they haven't read his latest book 'The God Delusion'- with chapter titles like 'Why there is almost certainly not a God' I think it is pretty clear Dawkins is not claiming certainty in anything.
The other spurious argument theists like Madeleine Bunting like to use (when they are not suggesting alpha courses are educational) is to say that God's existence or non existence are equi-probable - and of course they are right - IF you make no assumptions about the world whatsoever - but of course no assumptions mean my brain might be floating in a vat of tea and you might be a baked bean - these also become equi-probable.
The truth is, if we accept the reasonable assumption that sensory evidence is credible - like we have to when we cross the road - then God is extremely unlikely. It is as simple as that - end of story. If you want that argument in a little more detail then buy Dawkins wonderful book - The God Delusion - now in paperback.