He went from 246 to 262, not 270 as they have him listed there. Body fat went down from 12% to 10%. As fast as ever and even stronger.
As far as him being clean, the guy was in the weight room everyday when he couldn't practice. Working out. He hired a nutritionist, apparently for the first time in his career. Not to mention Luke Richesson is a world class strength and conditioning coach.
I think if you combine all of that, without the wear and tear of season eating away at your body, it's pretty easy to gain 16 lbs of muscle over a 3 month period. Plus if he was using PEDs, it's a huge risk for him. Could very well end his career or start the ending of his career. So I don't think it makes much sense.