Not even Dawkins would say that he "knows" that there is not god, no competent scientist would
The trick here is defining what "God" your saying you "know" isn't out there.
Here's the drill.
1) a theist brings up their very specific, theistic God. I'll use Yahweh as an example because I'm an American and Christianity dominates the religious demographic here. They talk about God having a son named Jesus, creating the world, answering prayers, judging people and eventually either damning them to hell or allowing them life everlasting in the paradise of Heaven.
2) an atheist says "I know there is no such God."
3) the theist, in an attempt to make this claim of knowledge seem unreasonable and arrogant says "Oh, so you're SO SURE there is nothing bigger than yourself? You're so sure there is no higher power or force we don't understand?
What has happened here is that the theist has backpedaled from the God they claim to know exists, to some vague deist sort of notion of "maybe something God-like that we don't understand."
Do I know there is no God that the theist claims to know exists? Yes.
Do I know that there is no vague deist sort of notion of "maybe something God-like that we don't understand?" No, of course not. This is the only kind of doubt that people like Dawkins leave. Dawkins isn't saying "Maybe Allah really exists and made people out of mud" or "maybe Yahweh exists and if you pray to him He will remove your cancer or help your team win the big game:" What he's saying is "hey, I don't know everything, maybe something incredible is out there that we don't quite understand.
There is a big difference between a God with a capital G who comes with a whole set of stories, and a deist notion of god for which people don't make any specific claims. You're right that no competent scientist would claim to "know" there is no such thing as the second one, but plenty will claim to "know" there is no such thing as the first one.
It's amazing in these kinds of discussions how rapidly people flip back and forth between the two definitions without even realizing it.