As a rule of thumb, never take anything Chael says seriously, as he knows nothing about most topics and is trying to make headlines for his videos and make bombastic statements almost always.
First of all, how much did Jon weigh before? Is there confirmation? I would assume he was about 225-230 in the off season, so that could well be 10lbs of mass, not 15. Second, he's not training for a fight, it's possible that if he goes into camp and incorporates more cardio and such he can drop some water weight to 235 or whatever. Third, those 10-15lbs are not all muscle. When you reincorporate carbs or start eating more in general, your body also retains more water and glycogen. And a part of that weight gain is inevitably fat. Lastly, I can have 2-3lbs differences depending on which time of the day I weight myself and I'm much lighter than Jon.
In short, it's hard to tell without body measurements and detailed before/after pics. We don't even know the exact starting/finishing numbers. And weight gain =/= muscle gain.
I do think Jon has juiced in the past. But there's nothing about those numbers that's necessarily suspect. His numbers in the big lifts from what I've seen are nothing crazy. It's not strange for an athletic guy who wasn't eating above maintenance to gain 10lbs in 2 months or 15 in 3 if he starts eating with a purpose, and stops depleting himself to make a weightclass.