You are correct in that a given number of track miles flown boards in/boards out the fuel usage is the same. However the fuel savings is in the early portion of the descent.
To get that excess potential energy at SIERA you have to burn more kinetic energy to get to SIERA at that height.
If you cross SIERA at 190 as opposed to 130 you have 6000 extra feet to descent. If you would have started your descent so as to cross SIERA at 130 you would have had to start down 18nm earlier(give or take depending on the airplane). 18nm at a GS of approx. 450kt is about 2 and a half minutes. That means that you are spendind an additional 2:30 at CRUISE fuel flow vs. an additional 2:30 at IDLE. That is a huge difference in fuel flow and thus fuel consumed.