I'm not using "matter" in a very deep way here. As a human, your decisions matter to yourself in the sense that you're going to experience the consequences of them. If you choose to eat an inedible object, then you're going to have a bad time. But if a Boltzmann brain chooses to eat an inedible object, it does not matter because the Boltzmann brain can't execute that decision in reality and is going to stop existing in a moment.
in the sense that we could be "brains in boxes" like in the matrix, one could argue that this reality we experience could happen to a boltzmann brain as well. similar to a dream, in which the dream reality is a result of feeding all the senses with data from within.
An important distinction is that a boltzman brain only "works" for an instant before the harsh environment of space destroys it. There's simply no time for a whole dream matrix reality to play out.
Another Boltzmann brain can pop up, which contains a short term memory of being previous Boltzmann brain. Sensory inputs aren't constrained though. Most probable sequence of such brains should feel like noise all over your senses, I think.
Why? The time passed for the Boltzmann Brain won’t necessarily be tied to its real frame of existence i think, it may simulate any timeframe imaginable, depending on its starting conditions.
In fact the simulation of any timeframe would be constrained by physical laws.
For example for a Boltzmann Brain with a similar chemical composition as ours the speed at which chemical reactions occur would limit the speed of experience.
For a Boltzmann Brain that simulates ours with a very different physical substrate the speed of light would still be a limit.
Why do you say that decisions matter if you're a human? At least, why would decisions matter more as a human than if you're a Boltzmann brain?