"No, but if a belief has, as part of a tenet, any religious content, then it's not science."
By Darwin speculating about a Creator, he involves something else in the evolution process aside from self-organizing life. This means that Creationism is suggestable as the origin of Life and the reason evolution works in his theory.
He holds that God is probable, thus by your assertion, it should not be considered science, as it makes a claim of a sigular higher being. (This would be considered religious as well, as there are people who hold that their are many gods, and some hold that their is but one.)