That comment by Wolfgang Langewiesche in his book "Stick and Rudder" has caused lots of discussion over the years. Maybe that is what he intended when he worded it so ambiguously.
It is really only true if you are flying just a little bit slower than best glide speed (best Lift /Drag ratio) or (best glide distance). Best glide distance means the speed at which the most distance is traveled for the least amount of sink. By putting the nose down and going faster, you will cover more distance as the aircraft sinks. Also, that statement is really only true if you point the nose up or down just a little.
If you are already flying at the best glide speed and point the nose down, you will increase the speed and distance traveled, but you will also increase the sink rate and drag form the higher airpspeed. That will result in a steeper descent because you are no longer at the best lift to drag ratio.
If you point the nose down to 60 degrees below the horizon I guarantee you will not descend less steeply.