Most of the corners where he's on the limiter are tight sections with consecutive corners. If he were to try and change gears, the moment he completed the shift from 3rd to 4th, he would immediately have to shift back down to 3rd for the next corner. Alternatively, if he were to lift off the throttle he would lose momentum and risk upsetting the rear end of the car. It's faster to just stay on the limiter, keep the car in 3rd through the whole section and keep both hands on the wheel to steer. This guy knows exactly what he's doing.