A Boltzmann brain requires a similar underlying reality except with no evolution traceability for the brain's origin.
Not requiring a world around the brain does not make it more likely, but less, as the probability of brains given that worlds exist multiplied by the probability of worlds is still much higher than the probability of brains regardless of the existence of worlds
You can’t even be certain that you aren’t a Boltzmann brain. If you could prove it (either way), it would be a publishable paper and could probably be the basis of a PhD for yourself.
Boltzmann brain is just one of the theoretically infinite combinations of matter that can be randomly conjured out of the quantum chaos (with a very low probability). Our entire visible universe could be a "Boltzmann Universe", and we'd have no way of knowing it.
If the Wikipedia page on Boltzmann brain is accurate, then the probability is surprisingly (but relatively) high.
> The Boltzmann brain gained new relevance around 2002, when some cosmologists started to become concerned that, in many theories about the universe, human brains are vastly more likely to arise from random fluctuations; this leads to the conclusion that, statistically, humans are likely to be wrong about their memories of the past and in fact be Boltzmann brains. When applied to more recent theories about the multiverse, Boltzmann brain arguments are part of the unsolved measure problem of cosmology.