1. A singularity happened in a human civilization, that was 1st to evolve in it's galaxy.
2. That civilization turned all the energy and matter in that galaxy into computronium.
3. They run a 10^70 simulations and we are inside one of those.
I don't think post-biological ET necessarily turns all the energy and matter in its galaxy to computronium either. It's possible that an amount of matter on the order of a planet or less is more than enough to satisfy. The same postulation can function as an attempt to explain the absence of Dyson spheres or anything of the like.
When we think of theoretical star-faring advanced ETs we anthropomorphize. We're imagining them on the same temporal and spatial scales which we occupy, but they are possibly many orders of magnitude smaller and faster. So this is a Kurzwielian view in that it extrapolates from the patterns that he identifies.