I agree. My problem is the term "computer simulation". We are making assumptions right away about what the words computer and simulation mean in this context. If a simulation is defined as something that has a set of "pre-programmed" reactions to stimulus then by virtue of the laws of physics governing our universe I would say that we are quite obviously living in a "simulation". The use of "computer" in this context is simply laughable, but it may imply (by the authors) that there is a processing outcome that the simulation owners are looking for, and the further implication that there are "owners" or "programmers". That would beg the question "What is outside of the simulation?" If it was a simulation and there was something outside of it, it seems very unlikely that we could ever detect whatever that is (to your point).
It seems much more likely that the set of rules responsible for our universe came into existence randomly and were such that our universe could and does exist. And that there exists many other universes with not quite the same rules and which would be unrecognizable (and undetectable) to us. And further that many would come into existence and collapse immediately because the parameters were not quite right to support existence.
This view is more in line with our observations of the evolution and iterative and ongoing changes in the universe we know. I believe you can make inferences about the larger whole from the behavior of the component parts.
"God does not play dice." I'm afraid that God does not exist, but the dice are real (my own immortal existence notwithstanding).