Let's consider another agent with which you consider cooperating as an instrumental installation, not valued in itself, but only as a means of achieving your goals that lie elsewhere. Of such agent, you're only interested in behavior. Preference is a specification of behavior, saying what the agent does in each given state of knowledge (under a simplifying assumption that the optimal action is always selected). How this preference is represented in that agent's mind is irrelevant as it doesn't influence its behavior, and so can't matter for how you select a cooperative play with this agent.