The benefit to the farms from using the lights doesn't come from feeding on the plankton or any of the other small fish species the study found were attracted to the light.
The benefit and the revenue accruing from it come from using photoperiod control to delay maturation and prolong the growth period supported by the regular day time feeding program. Once maturation begins, the fish divert the nutrients and energy obtained from the diet from growth to the formation of eggs or milt and no longer increase in size or value. As has been pointed out, though in manner that displays either broad ignorance of the subject or a deliberate intent to mislead, the farms can increase revenue appreciably by delaying the onset of maturation in order to extend the growth period.