Atheism is a worldview is something I have run across many times in discussions of religion. This is a variant of atheism is a religion. Religions are worldviews, or ways to think about the world. The idea is often advanced as a strawman when a religious apologist cannot justify the positions of his or her faith. With the inability to justify the contradictions of the Bible, Qur'an, Vedas, &c with observed reality, a strawman is created where atheism is a worldview and no different than Christianity (or insert any other religion here, because they all do it).
This strawman is not just used for atheism by apologists; apologists also use evolution is a worldview. (Evolution by Natural Selection is a scientific theory, not a worldview. For that matter, science is sometimes strawmanned as a worldview when scientific theories stand in opposition to a holy book.)
"Worldview" itself is a slippery word (and that's why the apologist uses it). The Oxford English Dictionary defines "worldview" in American usage as:
"A particular philosophy of life or conception of the world"
Atheism is not a philosophy or conception of the world.
By creating that strawman, the apologist then tries to use the most common fallacies in religious apologetics, shifting the burden of proof (see Apologetics Fail #4), and an equivocation fallacy ("belief" in atheism is exactly the same as belief in my god). The statement "atheism is a worldview" then attempts to put the burden of proof on an atheist to justify the position of lack of belief in any gods.
A lack of belief in gods shows exactly nothing about any other position an atheist might have. Someone stating "I am an atheist" says absolutely nothing about anything else the atheist knows or believes. It has no philosophy of life or a conception of the world. Atheism is precisely not a worldview, in the same manner that abstinence is not a sex position, not collecting coins is numismatics, bald is not a hair colour, &c. It is the answer to a single question, do you believe in a god or gods (no).