If the observable universe is too small, what about the observable universe sampled at multiple points in time? Or just an iron bar that's repeatedly sampled along a time interval, with the desired encoding changing between each sample such that the object doesn't need to undergo much change it state in order to map to distinct things. Each sample increases the number of physical states that we can create a mapping from. If we still don't have enough states, we can draw more samples over more points in time to create a longer string of bits for the mapping.
Or another way to do it would be to repeatedly sample the same object at the same point in time, and simply change the desired encoding on each sampling, e.g. on a first pass we use F_0(x), on a second pass we use F_1(x), and we concatenate the output of F_0(x) and F_1(x) into a single large bit array.
The Boltzmann Brain says that our conscious experience at the current moment is a chance quantum event. It might not even involve the whole universe, it could be a local quantum phenomenon. That seems distinct to the argument in the article, which is that physical states (quantum or otherwise) can be mapped to an arbitrary string of bits with the encoding chosen by the person doing the mapping, and the output of this mapping (which is itself a string of bits) can correspond to a computation on a Turing machine, therefore inanimate objects with sufficiently large numbers of physical states could be said to be running a proposed consciousness.exe, which leads us to an absurd/extreme form of panpsychism.