I can attest that comment is right on target.
When I went to the chicken slaughter, I learned very quickly about the parts that needed to be removed. Since I went primarily to glean bits for pet food (heads, organs), the other folks there kept asking if I wanted the gullets and intestinal tracts. After close examination, I decided against them. There was too much probability of rocks and feces left in them. Cleaning out the giblets was educational as well, they were full of grass, hay, silt, and pebbles.
So, yeah... if there are rocks in the food, that says there was a breakdown in the cleaning of the meats in the first place.