I believe that is correct for the M48A3-M60A3. Selecting reverse just reversed the direction of low range giving them a theoretical reverse speed of 9 mph. The M1 does the same for 1st and 2nd gears giving it a 20 mph reverse speed.
I always thought it was the same as low gear forward which was 9 MPH on the M-48s.
I was always struck more by how fast an M1 unit could displace more than the speed of an advance/attack.
Not being around tanks much in ROTC, other than seeing them roll down tank trails or in branch day, and then in the Armor School (and the usual newbie clusters while maneuvering), it wasn't until arriving at my first unit and doing company lanes that I realized the violent speed in withdrawal as platoons pulled out of line, and a whole company repositioned.