I think they're being "fast and loose" with the term "bag". Like *all* animal feed, the Mill produces 1 "item" that is "3 bags of feed / substrate". When collected, each "item" takes up 3 slots in the barn so from that point of view, collect 1 item, get 3 bags. So 2 "collections of items" from the Mill yields the 6 "bags" needed for a full mushroom field.
So I believe their use of "2 bags" in this context is incorrect, they really mean "2 Mill shelf items --> 6 bags"