In economic theory, the term “preference” has a narrow, technical meaning. It refers to the rank order of the two alternatives a person chooses between in taking an action.
In common discourse, “preference” has a broader, non-technical meaning. It refers to what a person favors.
For example, out of all the means of transportation, I favor a Mercedes S-Class Sedan. However, I prefer a Honda Accord. We know this because I own an Accord.
Conclusions about policy issues are not preferences in the narrow sense. They are the last step of an argument. When someone states that he favors or prefers open borders, he is saying that his analysis reaches this conclusion for the various reasons outlined in the argument he makes for open borders.
It would be a mistake, as you point out, for a person to assume that everyone else must have the same preferences that he has for alternatives in taking an action. For example, for me to assume that everyone must prefer, and hence own, a Honda Accord just as I do. It would also be a mistake for a person to assume that everyone else must be persuaded by an argument that he favors.
A person cannot be indifferent between the alternatives of choice in his own action. To act, he must choose and to choose, he must prefer. A person, however, can be indifferent about policy issues or the actions of other people or even his own potential actions.
While a person can be indifferent about policy issues, advocating a policy is an action and thus, requires a choice and a preference. I don’t see why a person couldn’t advocate against all state activity even if he thought that open borders was a good policy. Whether or not his position was logically consistent or wise would be an open question, but it would certainly be “rational” in the praxeological sense.