If you believe in a Matrix or in the Simulation Hypothesis, you believe in powerful aliens, not deities. Next! ... This is mere distortion of both the common informal use and advanced formal definitions of the word "atheism", which is not only unhelpful but such a common religious tactic that you should not be surprised to be downvoted.

It bothers me when an easily researched, factually incorrect statement is upvoted so many times. There are many different definitions of atheism, but one good one might be:

An atheist is one who denies the existence of a personal, transcendent creator of the universe, rather than one who simply lives life without reference to such a being. A theist is one who asserts the existence of such a creator. Robin Le Poidevin. Arguing for Atheism: An Introduction to the Philosophy of Religion

The book does not define personal or transcendent, but it is unlikely that either would exclude "god is an extradimensional being who created us using a simulation" as a theistic argument. For example, one likely definition of transcendent is:

transcendent: the realm of thought which lies beyond the boundary of pos­sible knowledge, because it consists of objects which cannot be presented to us in intuition-i.e., objects which we can never experience with our senses (sometimes called noumena). The closest we can get to gaining knowledge of the transcendent realm is to think about it by means of ideas. (The opposite of 'transcendent' is 'immanent'.) [http://staffweb.hkbu.edu.hk/ppp/ksp1/KSPglos.html]

Beings living outside the simulation would definitely qualify as transcendent since we have no way of experiencing their universe. To be clear, I am not saying this is the only possible definition of atheism. I am only saying that it is one reasonable definition of atheism, and to claim that it is not a definition, as Eliezer's post has done, is factually incorrect.