That question only makes sense in reference to the model of the universe you choose and you estimate parameters. It's not a precise argument, it's a warning about theories that make life too improbable.
And of course probability doesn't matter for events that occur only once in the universe. This isn't a scientific theory.
I take the boltzman brain convo as one that takes BBs as forming within the universe we are familiar with, and thus grounded in probabilities based on physical interactions of the kind we know about that happen in our universe. A BB could spontaneously form out in space, and the rules of space, entropy, etc are things we are familiar with. We can make reasonable estimations of the probability of things that happen within our universe. A boltzman brain forming within our universe is one thing, and life forming is another.
And if we've thrown that core reference frame out the window we've stepped entirely outside of the rules that are the basic structure of the whole boltzman brain thought experiment.
And of course probability doesn't matter for events that occur only once in the universe. This isn't a scientific theory.