It clearly would be irrational for someone to say: “I believe Gods do not exist, but I believe God is the basis for morality!” (or for the lacktheist “I do not believe God exists, but I believe God is the basis for morality!”).
Even if some lack of belief atheist (which is actually called “agnostic” in academic literature) maintains their “lack of belief” is not a worldview, the same argument applies. Their failing to affirm the proposition God exists *precludes* them holding the belief God is a causal factor in explaining the universe, *precludes* them from believing that God is a causal factor in explaining the existence of the universe, and *precludes* them from believing that God is the basis of morality. The very preclusion of these beliefs by epistemic commitment, inherently affect a person’s overall perception of reality…which is typically, at minimum, what is necessarily considered to be constitutive of a worldview.
Disbelief, or unbelief, of God existing or not, infers other epistemic commitments. If theism is construed to be a “worldview” as how a person views reality, then atheism as well must be considered a “worldview”, or why bother having that particular sense of the word “worldview” at all?