you shouldnt need additional lighting. about the only time you would need additional lighting in a greenhouse would be to extend the light hours, in the case of cannabis that would be used to keep it in a veg state if you did not want the plants to flower. would probably only be needed in your case in the winter or very early spring when the days was less than 13 hours.
the reason he is blacking out the greenhouse is so he can go 12/12 in june its a daily process where the green house is covered to provide darkness so the plants will start to flower alot early than they normaly would, its not uncommon to do this in a green house.