Sugar or Stevia neither would cause that problem. It might have been not enough baking powder. I find that it takes about 4 teaspoons to 2 cups of flour to make biscuits rise so I am thinking it should have been more. Also beating for 10 minutes will take some of the action out of the bp.
And yes, that is way too much sugar. I would have adjusted it to 1 cup total. For only 2 cups of flour 1 cup of sugar would have been sufficient. Also the recipe uses yogurt. In that case I would have used 1 teaspoon of baking soda so the reaction would be more lift. I would have added the baking soda toward the end of mixing just enough to incorporate it.