It reads to me like the author is stating that a stiffer material means it could make an actuator with more output power, which doesn't make a lot of sense to me.
Maybe they mean a more efficient actuator? Actuator output power is just any definition of mechanical power; either a force times the linear speed or a torque times the rotational speed.
A stiffer material may mean that the input power is transmitted more efficiently to the output end of the actuator because the motion isn't "consumed" by flex or deformation in the actuator material.
More efficient actuators then would imply that you could make an actuator that had the same weight but a higher output power, but that's specific power, or the power-to-weight ratio. This may be what the author meant - stiffer materials could lead to actuators with greater specific power output.
You should try to contact the author(s) for more information! I've only tried contacting authors a couple times, but I've gotten really friendly responses every time I have.