I can refute that claim: I went to Marine Corps bootcamp (PI SC), weighing approximately 127 lbs. I left (3 months later) weighing 142lbs - 15 lbs in 3 months. I think if I had gone on double rations I could have gained more.
It's also important to note that there's a difference between dry muscle mass and what you read on your scale after you've taken away body fat (that's presumably "wet muscle" - I don't know the exact word for it). So in your case, you might've gained a bit of fat (let's say 2 lbs for sake of argument), which means you gained 15-2/3 = 4.3 lbs of "wet muscle", which is entirely possible according to this article. You'll definitely gain more weight if you doubled your rations, but I suspect that will shift your muscle-to-fat gains ratio much more towards gaining fat.
I've also made similar gains at one point, going from 140-155 in 3 months. I did this through a diet of ~3600-4500 calories, and I gained a bit of fat too probably from overshooting my caloric intake near the end at 4500. Granted, my diet wasn't perfect, but it was pretty regulated so that's why the original claim's rate of muscle gain draws my attention.