Why? As I understand it, Boltzmann-Stuart affects its own world, in the same sense as Harry Potter affects Hogwarts but doesn't affect Mordor, it's control in a hypothetical situation. We don't require that any part of the setup is real, instead we see whether the agent controls the environment inside the hypothetical. And its own world has the same kind of valuable people as your world, in the same way as Albus Dumbledore is a morally relevant feature in his hypothetical world, while a non-magical chair in Hogwarts isn't. The problem with Boltzmann-Stuart's world is not that it's not affected by Boltzmann-Stuart, or that it doesn't have anything of relative value in it (compared to the value of the world itself).

The problem, from your perspective, is that the whole world is in some sense "not real". For the purposes of decision making, being "not real" seems to be the same thing as not being morally relevant, hence the enumeration of possible reasons for not being considered relevant in my comment. The reason that seems to apply in this case seems to be the measure of caring over possible worlds, which I guess depends on which world it's originally formulated from.

Another hypothesis is that you don't identify Botzmann-Stuart and the hypothetical Stuart that lives in the hypothetical world that Boltzmann-Stuart merely believes he's inhabiting. In that case my argument can be seen as being about the hypothetical Stuart, rather than about Boltzmann-Stuart, with the additional assumption that the hypothetical Stuart should be identified with Boltzmann-Stuart (which I didn't argue for). I identify them is a way similar to how it's done in discussions of UDT and counterfactual trade. For example, we may consider hypothetical Stuart a predictive device that manages to enact the decisions of Boltzmann-Stuart, so that any decisions made by Boltzmann-Stuart affect the hypothetical Stuart's world.