It's the real deal. I have had a couple of surgeries and other incidents that have kept me out of the gym for months at a time and experienced some significant atrophy over those time periods. Things start out slow once you actually get back in the gym but the weight, strength and size comes back quickly once you start training and eating right again. It seems like after a month or so I am back up to where I should be again.

I believe there is legitimacy to the concept of muscle memory. there are a few different ideas and concepts behind it. one being that your body is family with the motor patterns which plays a huge role in the neuromuscular connection. this can lead to faster gains when you return.
the second factor is that previous hypertrophic growth is hugely beneficial from the past because it resulted in an increase of myonuclei which don't degrade or "shrink" as muscle fibers can when gone untrained.
ultimately, as hooker said below, get in there and push hard. you should get faster gains that the average untrained newbie, assuming rest and diet is in order.