From the analysis of the report I don't see that the root cause was actually identified.
It is not clear that the metering valve piston was actually seized. It may has been tight due to the observed deposit of SAP material in the fit clearance so it would be difficult to move it with hand pressure but this does not mean that the torque motor - servovalve should not be capable of moving the stuck piston - if the servo valve was functional. I don't know the exact numbers but with a servo supply pressure difference of 1000 psi and a 2" diameter piston at least a few hundred pounds of operating force should be available to move the piston and maintain some kind of partial metering control.
I think it is likely that control was totally lost because the torque motor - servovalve was plugged and according to the report nothing was done to evaluate condition of this component.