Ditto to everything maxawaytoolong posted
TLDR: Mostly lost water weight. I'd done it enough to really know how my body worked. I knew the exercise techniques that worked particularly well for me, and I ate a super-clean diet.
This wasn't a Jared Fogel style "I lost x lbs." He lost fat/tissue. For wrestlers, it's mostly a water cut. In sports like wrestling, boxing, or anything with a weight division you may need to lose water weight to get down to the cap of your division. I was 171 in HS and 197-184 in college. You have to be below the division cap (any over 171 in this case would get you DQ'ed).
Water weight isn't too hard to lose when you start. In your sleep, we all lose from 1-3 lbs, then we use the restroom when we wake up (.3-.6 lbs). If you start with a workout in the morning (1-2 lbs depending on the workout), you can walk 2-3 more lbs off throughout the day. Of course, normal people eat and drink in the course of the day to "refill." Over time, you get to the point where you know how to master this cycle, with extra workouts to boost it, and control your diet to engage the metabolic systems. It's a delicate balance.
In college, I wasn't a starter who had to weight in weekly, so I wasn't always close to weight. Even though we'd work out 2-3 hours a day, we have our bodies to the point where an extra push is required to get down to size. Typically, someone my size and structure should never have been more than 5-10 lbs overweight at most. I just came in one Monday weighing a great deal, and I was also tapped to start for that match to give our starter a rest. Since my resting/homeostasis weight was 5-10 lbs over, losing the first 10 lb chunk was easy. The next portion was a little rougher, but I'd been doing this for 7 years (HS + college), so I knew how to do it.
As for the techniques, I would run, jump rope, and wrestle wearing sweats. Wrestling has a higher caloric exertion rate than boxing, which is already way higher than running or jumping rope alone. Our college facilites also had a steam room, so I would clock time there to keep my sweat up after finishing a workout.
As for diet, I'd start each meal with a couple big glasses of water, so I wasn't quite as hungry and wouldn't binge. I'd then eat nutrient-dense vegetables (spinach salad, bell peppers, seeds, berries). Then eat lean protein (chicken breast, fish, no beef). To end I'd either drink a V8 or a yogurt shake. After all of that water and nutrient dense food, you feel stuffed.
All this being said, you'd have to be completely moronic to try to rapidly that much lose water weight without having done so before. Do. Not. Try. This. At. Home. Trained athletes have died doing this wrong