The interesting part of the hypothesis, as I understand it, is less that the probability of a Boltzmann brain approaches one as the universe grows older (trivially true) and more that the amount of negentropy needed to generate a universe is vastly, sillily larger than that needed to generate a small self-aware system that thinks it's embedded in a universe at some point in time -- and thus that anthropic considerations should guide us to favor the latter. This is of course predicated on the idea that the universe arose from a random event obeying the kind of probability distributions that govern vacuum fluctuations and similar events.