Yes, it is necessary. It's probably just not a step added to the recipe, as it's just something that people do with packaging, unless they keg.
I like 1 ounce of corn sugar (by weight) per finished gallon of beer. I "lose" lots of beer to dryhopping so when I make Pliny, I end up with about 4.25-4.5 gallons of finished beer if I make a 5 gallon batch!
I believe that's why the original recipe calls for 8 gallons out of the mash with 5 gallons after fermentation. To account for evaporation and dryhopping. This recipe seems to be a bit off in both gravity and color.