I think the point is that, if probability is only in the mind, the analogy between averaging over future yous and averaging over other people is weaker than it might initially appear.

It sort of seems like there might be an analogy if you're talking about averaging over versions of you that end up in different Everett branches. But if we're talking about subjective probabilities, then there's no sense in which these "future yous" exist, except in your mind, and it's more difficult to see the analogy between averaging over them, and averaging over actual people.