Yeah this video is...not great. The point of Boltzmann brains isn't that you might be one. Boltzmann brains aren't just some weird cosmological version of the Cartesan doubt question of, "Am I just a brain in a vat being fed information by a computer?" Nor are they weird cosmological verson of last Thrusdayism.

The point is that, if we accept that the universe isn't completely fictional, then we're obviously not Boltzmann brains. Which means (or so the argument goes) that the big bang was not a random quantum fluctuation out of a previous cold and empty universe in a state like what ours will reach in the distant future. We are not in a cyclical cosmology of big bangs and heat deaths.

Why? The reasoning goes, that if we were in such a cyclical cosmology, then most observers would find themselves in universes with the highest possible entropy that still allowed for observers, because high-entropy universes are much more likely to fluctuate into existence than low-entropy ones. Which is too say, in a cyclical cosmology we would expect most observers to be Boltzmann brains. But we aren't Boltzmann brains. The entropy at the time of the big bang was far lower than it needed to be to make observers.