On one hand, the belief in a god doesn’t just end there. There are beliefs in what that god does and what he has control over. So it’s completely logical to believe that there’s no god (although, as someone else pointed out, it’s also not random arrangements of atoms).
On the other hand, simulation theory is a logical theory to rationalize the “purpose” of why we exist. It’s not a belief. The simulation doesn’t respond to prayers or requests. It’s simply conjecture or hypothesis to explain the “why” of the universe. No one who talks about simulation theory (much less who “believes” in it) pretends that the creator of the simulation is uniquely interested in them and responds to their requests and tells them how to live their life. In fact, that would go against the entire concept of simulation theory.
Religion and religious belief have specific definitions. This feels just as dishonest as people claiming that LGBTQ ideology is a religion or that evolution is a “belief”.