Could someone be an atheist activist if this person does not try to persuade others than atheism is correct? Yes. This part of Greta Christina's definition does not strike me as essential for atheist activism. One could be an atheist activist without attempting to de-convert others.
Similarly, if asked whether someone could be an atheist activist without "working to change the world into one without religion," I'd likely say yes. While most people I would consider atheist activists certainly work to reduce the political influence of religion, I do not think that they necessarily must seek the abolition of religion.
In addition, I suggest that one of the ways I often use the term "atheist activist" is not captured by the above definition at all - one who works to promote atheist equality. However, while I consider this an important form of atheist activism, I do not view it as an essential part of the definition.
So what is an atheist activist, and what is atheist activism? I'd like to suggest something along these lines:
Atheist activism refers to the process of promoting atheism through activities such as promoting a worldview free from gods, reducing the privileged status of religion in society, and promoting atheist civil rights.Use of the "such as" phrase reminds us that these are merely examples of how an atheist activist might promote atheism, none of which is a necessary condition. The essential feature would be the promotion of atheism (i.e., I cannot think of a form of atheist activism that in no way involves the promotion of atheism).