If you notice, this brownie recipe does not call for baking soda or baking powder, yet the batter rose as it should.
Why is that, you ask?
Because the eggs act as a leavening agent. Mixing the sugar and eggs for 10 minutes adds air and creates a semi-stable mass of bubbles. Not as many or as stable as just beating egg whites, but the incorporation of air into the egg allows the brownie batter to rise as if chemical leaveners were used.