I feel obligated to point out that Boltzman brains aren't a hypothesis themselves, but a counterargument to the hypothesis that the current low-entropy state of the universe is a random low-entropy fluctuation in a steady state high-entropy universe.

The idea is that a brain randomly forming out a maxentropy soup is a lot more likely than enough negentropy to comprise our entire universe, so if you believe in the steady state hypothesis, then you have to conclude that you are far more likely to be a bloltzman brain, than not.