Those usually on a calorically deficit diet of usually 500 calories are usually eating very well and taking very good multiviatmins, and supplments like Whey Protein, and Glutamine only help. I would say it woul be a big plus, if you are eating very healthy even with a calorically deficict diet you are not only getting into better shape but your body as a whole has been super charged it will run better, faster, and stronger, and this evne comes down to its ability to repair the body, and those microtears that you place on your pee pee.
Is a great diet crucial for gains? No, but it doesn't hurt at all. I know DLD in the past has been on the whatever I want diet, but I see he is losing weight and eating better, and if his gains jump up in speed I wouldn't be surprised.