I've read interviews with Casey where he described the particulars of this experience. He had been in an accident and hadn't been able to train. Arthur Jones contacted him about the "experiment" and offered him a large cash incentive for every pound he was able to "gain." Casey basically starved himself to get into the lower 160s and then did the experiment training brief and super intensely while eating a high fat "whatever-he-wanted-to-eat" diet. Read as: Starved himself and didn't train for months, then pigged out and trained hard. Were the numbers legit? I don't know. But, to be 212 lbs as a mesomorph and not very cut isn't totally unreasonable to accomplish.
I competed in my first contest awhile back and weighed 197 on the day of the show, by the night after the show (about 30 hours later) I weighed 210, 13 lbs gain in 30 hours. Did I pig out? Yes, but didn't force feed. Mostly it was carbs and water weight. That's what happens when you starve and dehydrate then rehydrate and pig out.
Jones skewed the actual circumstances of this "experiment" in order to sell his nautilus equipment. But, knowing the true details, the whole thing makes a lot more sense. Muscle memory? Kinda, more like recovering from and accident with lots of food and heavy training. Maybe juice, who knows but Casey himself? I'm just saying he basically regained what he already had in a short period, however he did it.
Jacob