Rather than saying that the authors presume the problem of defining human interests has been solved, I would say that the authors are talking about a problem that also has to be solved, separately from that problem.

If we want to drive to the store, we have to both have a working car, and know how to get to the store. If the car is broken, we can fix the car. If we don't know how to get to the store, we can look at a map. We have to do both.

If someone else wants to use the car to drive to church, we may disagree about destinations but we both want a working car. Fixing the car doesn't "presume" that the destination question has been solved; rather, it's necessary to get to any destination.

(OTOH, if we fix the car and the church person steals it, that would kinda suck.)