Ion thrusters achieve substantially higher specific impulse than chemical thrusters like hydrazine by electrostatically accelerating propellant to high velocities, which is a well-established foundational concept in spacecraft propulsion.
The claim compares the specific impulse of xenon ion thrusters to hydrazine chemical thrusters. This is a foundational, textbook fact in aerospace engineering; electric propulsion inherently operates at much higher exhaust velocities and specific impulses (thousands of seconds) than chemical monopropellant systems (typically 200-300 seconds). No citation is needed for this basic physical fact.