Its worked out using a 600w hps. How they get to that figure is likely SOG so they can state the highest figures.
But in reality its a nonsense figure. It doesn't help you with a single plant. What you need to do is focus on filling your space with whatever plant count you plan and keeping it controlled by utilising training methods.
By controlled I mean shape your canopy so all of it gets a good even amount of light. That will help you get the most from your space.