This is accurate. A donut is a wall of baked dough around a hole (ignore jelly-filled doughnuts). The same is true of triploblasts, which consist of a digestive tract (a hole, albeit with muscular sphincters to pull its sides together at certain times and places but not to truly seal it off) surrounded by tissue.
Work Step by Step
To see this, remember the consideration of the digestive tract as the outside of the body. One could see, however strange, that these organisms are then essentially ring-shaped similar to a doughnut.