Human minds are fairly free to believe untruths. At least to a certain extent: it's rather hard to _really_ believe things that contradict your lived experience.
You can _say_ that you believe them, but you won't behave as if you believe them.
The problem with Boltzmann brains is that, by construction, they're going to have incorrect beliefs about almost everything.
Like, imagine watching a TV tuned to a dead station and somehow the random background noise looked and sounded like someone telling you the history of the world, and it really was just random noise doing this — that level of being wrong about almost everything.
Not even just errors like believing 1+1=3, but that this is equally likely as believing incoherent statements like 1+^Ω[fox emoji].
You can _say_ that you believe them, but you won't behave as if you believe them.