If I borrow a generator from a neighbor, then I am morally obligated to return it when my immediate need for it is over. But suppose, instead, I borrow a car, and when I go to return it, the neighbor from whom I borrowed it is very drunk and wants to drive the car; then the obligation to return the car immediately is much less clear. So not all cases of borrowing are equivalent.
The main point of the argument is that although having borrowed something gives rise, in an ordinary case, to an obligation to return the item promptly, it does not so obviously give rise to such an obligation in every case.