It's a nice way of putting it, "the nickel comes for free", because of the other metals. But it's not more thant a way of making a calculation, an abstraction. Or in fact kind of a joke? For in this scenario a cost is now attached to the other metals, and you might equally like to reverse the calculation and claim that these are the ones which are for free... Eventually, a serious way of stating things would seem to be: the mixture of different metals makes the project economically very robust with respect to nickel prices. It's basically not more than this, but not less, either.