This is a point of confusion I still have with the simulation argument: Upon learning that we are in an ancestor simulation, should we be any less surprised? It would be odd for a future civilization to dedicate a large fraction of their computational resources towards simulating early 21st century humans instead of happy transhuman living in base reality; shouldn't we therefore be equally perplexed that we aren't transhumans?
I guess the question boils down to the choice of reference classes, so what makes the reference class "early 21st century humans" so special? Why not widen the reference class to include all conscious minds, or narrow it down to the exact quantum state of a brain?
Furthermore, if you're convinced by the simulation argument, why not believe that you're a Boltzmann brain instead using the same line of argument?
I don't believe that that is a necessary assumption at all; the conscious state is still an abstractable representation, and if it maps to a dynamic process that itself can map to a temporally-connected collection of brain-states, then that is just more layers of abstraction.
The Boltzmann Brain could easily be not a brain-state representation, but a conscious-state representation.