The paper argues that any reactionless drive that produces more force per newton than a perfect photon drive can be turned into a perpetual motion machine, regardless of how it's built:
"Since applying a constant force results in a constant acceleration, the kinetic energy of a mass driven by such a device increases quadratically with time, while the energy input increases only linearly with time. Thus, at some point, the kinetic energy of the device-driven mass exceeds the energy input, and if this energy is collected via decelerating the mass (via regenerative electromagnetic braking, for example), then there would be a net gain in energy."
With a photon drive, the break-even point is at the speed of light (which you can never actually achieve), but with any drive that's more power-efficient, the break-even point will be less, and you will be able to extract limitless free energy by accelerating the drive past that point.
"Since applying a constant force results in a constant acceleration, the kinetic energy of a mass driven by such a device increases quadratically with time, while the energy input increases only linearly with time. Thus, at some point, the kinetic energy of the device-driven mass exceeds the energy input, and if this energy is collected via decelerating the mass (via regenerative electromagnetic braking, for example), then there would be a net gain in energy."
With a photon drive, the break-even point is at the speed of light (which you can never actually achieve), but with any drive that's more power-efficient, the break-even point will be less, and you will be able to extract limitless free energy by accelerating the drive past that point.