Why is the opportunity cost 0 in this case?
I think the point that the author was trying to make was that opportunity cost refers to the "best alternative use" of something. The measure is used to understand marginal decision making. Suppose you have just purchased an Oven which can either bake cakes or bake bread. Now if you decide to bake cakes, your opportunity cost is the number of loaves of bread that you could have baked at the same time. However, all this analysis is done keeping in mind that - "once the oven is purchased". 

The machine in your example has no alternative use and hence has zero opportunity cost. Your analysis about the interest is regarding the money using which the machine was purchased. Sure that money could have been put to some other use - like buying a Netflix suscription or putting into bank deposit and hence will have an opportunity cost. Bust as far as the machine is concerned " once it is purchased", on the margins, it has no alternative use hence no opporunity cost.