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 the rule that one ought to return to others what one borrowed from them is an exceptionless rule, though some cases may seem to be exceptions.