The quote also doesn't say what you're claiming it does. Saying that the history of evolution does not necessarily require the design by or direct intervention of God is not the same thing as claiming that evolution disproves God's very existence.That quote was from his autobiography:
And Darwin's personal views are in a large part irrelevant when it comes to the arguments that he made. His beliefs certainly informed his writings, but they can be judged for what they actually say.
In fact, the point that you made in another post, i.e. that his children hid his feelings on Christianity until after he died, seems to indicate that they would have changed public opinion of Darwin. Whatever his writings said, this action certainly seems like Darwin's extant writings weren't generally taken as some sort of "argument from indesign" for the non-existence of God. If Darwin had actually been shouting "God does not exist!" from his soapbox, any misgivings he had about Christianity wouldn't have been news.