They don't know for sure, but this is the hypothesis from the paper: "Anatomical connections may have persisted after the injury that were previously 'silent' because of loss of conduction as a result of disruption of myelin or the ionic channels of the neurons."

Basically, the electrostimulation is allowing damage neurons to function.

It's indeed a bit weird, but I think it reenforce the idea that the spinal chord is an extension of the brain, not just a communication channel.