I think they were imagining something more like a bakery / brewery / chemical plant that would turn grasses into edible bread / porridge / slop.
Well, you've got a chemical plant that can turn tough cellulose-y grasses and leaves and heather into stuff we can eat, it's called a sheep.
How you'd actually get your stomach to brew that up into anything useful in time is anyone's guess, and what it would do to the rest of you is an exercise for the keen experimenter.
We are basically too active and too large to eat grass, even if we had lactase.