Sugar improves browning, gives bulk, AND flavor. In making jams and jellies, it is what reacts with the pectin to make the stuff gel. I'm all for stevia, but it is not interchangeable. In that apple cake recipe, the extra sugar was probably in there to caramelize, resulting in a caramel-apple flavor that stevia can't give you.
Were the apples particularly juicy? Some varieties are better for baking than others.