I don't know if we live in the simplest universe giving rise to life for some philosophically and deeply "correct" notion of simplicity. However, I don't think this is needed for the argument in my post you are asking about. In fact while writing that post I was implicitly assuming that the attacker's universe is about as complex as our own, in order to make my argument harder.
First, I think that if we write down a particular universal prior (e.g. by choosing a universal turing machine like a Python interpreter), then we probably won't be the simplest:
- The actual simplest universes will be different across different programming languages. This is plausible because those universes are themselves simpler than implementations of universal Turing machines without preprocessing. But it's very hard to really know.
- If you believe this, then it's unlikely that we have the simplest universe according to say the Python prior, or any other concrete prior we write down. There's just one "right" prior according to which we are the simplest.
- Even if that prior were philosophically distinguished, that doesn't help us unless we do that philosophy and pick out the right universal prior.
Even if we did live in the simplest universe according to the chosen universal prior, it seems like we'd probably get a simulation:
- The anthropic update (including the inference from the language choice) is a huge advantage for simulators.
- It doesn't change the story that much if we get a simulation from a universe with our physical laws vs other physical laws.
- The awkwardness of reading out bits, and ensuring that evolved life has maximal control over that channel, probably puts you somewhere other than the absolute simplest universe.
Aside from the arguments I had in mind while writing the post, there is a more philosophical reason (that I've thought less about) to think that most early civilizations we care about aren't living in the simplest universe:
- I expect that "most" early civilizations are in "dense" universes, at least if you try to weight them by their intrinsic moral worth.
- I expect that it's simpler to create truly humungous simple universes with a lower density of life. Note that the complexity differences we are talking about here are very very small.
- Some of those universes will still allow consequentialists to control arbitrary output locations, despite starting from a very low density (e.g. faster than light travel would be very helpful).
That said, I do think that a lot of future influence comes from huge simple universes with easy travel, even if most early civilizations (by moral weight) aren't in such universes. And if you care about the moral weight of our civilization itself then I think it is plausibly dominated by the simulations, such that weighting by "future influence" is the only real weighting that's meaningful to apply to early civilizations.