The boltzmann brain arises from a statistical fluctuation. I think he is talking about the simulation hypothesis and in particular arguments put forward by the philosopher Nick Bostrom.

He mentioned the likelihood of existing in a simulated universe in the context of probabilities - the Boltzmann Brain is the construct I know of that directly addresses this.

Bostrom's argument is also a statistical kind of argument. I can't know what Daniel was thinking - I merely hooked on the words most likely living in a simulated universe. and all of our sensory inputs are being manipulated in ways that mimic the universe we think we live in.

The cause of a Boltzmann Brain does not act directly, it's merely a random fluctuation that allows an arrangement of particles to form a mind that thinks it's existing. There is no simulation or manipulation or attempt to fool. It just is something so unlikely, you have to wait an unmentionable amount of time to have it occur. Assuming there is nothing in physics that ends up barring such.

The Boltzmann brain argument is one that is hard to swallow and very unlikely by definition. Bostrom's simulation argument puts forward that we are most likely in a simulation and makes a compelling case.

The Boltzmann brain is about a conscious entity arising due to chance, or the known universe for that matter; cf. http://en.wikipedia.org/wiki/Boltzmann_brain The word simulation does not occur on that page, it is about our universe being just a fluke.