If I were making "meatballs" that I wanted to stick together through simmering in spaghetti sauce, I personally would use some vital wheat gluten as a binder. That would give you a meatball that wouldn't fall apart if simmered in sauce for a while, and it would also give you a "meatier" texture, especially if combined with something like the Lightlife burger crumbles used in the one recipe, or finely diced mushrooms that have been sautéed with onions until their liquid has been reduced (in the latter case, I would also add breadcrumbs to the "meatball" mixture).