Not strictly true if my memory serves me right. I freely admit that my manuals are up in the attic and that I am 20 years out of date but I seem to remember that we had reversible and non-reversible motor pumps in the flying control part of the hydraulic systems.
The non-reversible motor pumps had their own reservoirs which still contained fluid in the event of the failure of the main hydraulics.
The theory was that these reservoirs would have contained enough fluid for the automatics to work the control surfaces according to my mentors at AA in DFW.
Incidentally, we used to practice flying the DC-10 in the simulator just by using the engines after the Chicago disaster (a long time before Sioux City). Their biggest problem was the fact that they didn't have No.2 available to control pitch.
We were taught to use Nos 1 and 3 for turning.
With No.2 available it was a relatively easy exercise.