If you look at downforce wings on some cars you'll see a similar but one sided lip they call a "Gurney Flap" after Dan Gurney, the racer that introduced these lips to car racing. The Gurney Flap is supposed to provide more lift but it's not as good as just using more camber. Selig tested such flaps in his early windtunnel work and from what I saw that was the outcome. More lift but far more drag that could be obtained with more camber. It's possible that this lip on the rear of the rudder is there to get more response out of the rudder if it's already at max throw and limited by structural factors. Or it may be there to soften the response as suggested. Either way it would certainly add drag