(Though I think the paper was trying to use dispositions to define "rationality" more than to implement an agent that would consistently carry out those dispositions?)

Unless "cultivating a disposition" amounts to a (subsequent-choice-circumventing) precommitment, you still need a reason, when you make that subsequent choice, to act in accordance with the cultivated disposition. And there's no good explanation for why that reason should care about whether or not you previously cultivated a disposition.

That's just what "dispositions" are in this context - tendencies to behave in particular ways under particular circumstances.

By this conception of what "disposition" means, you can't cultivate a dispositon for keeping promises - and then break the promises when the chips are down. You are either disposed to keep promises, or you are not.