No, he is literally adding a gallon of ground coriander to each 15 bbl batch. 15 bbl = about 465 US Gallons. If you have ever worked in a restaurant, a lot of spices come in huge gallon containers. He is probably buying something like that through a restaurant supplier. If I were you, I'd look at a few homebrew recipes and figure out how much people typically use instead of trying to scale that down. You are probably looking in the 1-2 oz range.