It's probably 25 pounds of weight, with some clear and significant muscle growth. If you just said someone gained 25 pounds it might be interpreted that they ate cheeseburgers and donuts all winter. So you clarify by saying "that's muscle, not fat". Does that mean you are saying that every ounce gained is muscle? Of course not. You are making the point that there is clear, visible added musculature that accounts for a significant amount of the weight gain, that's all.
People took that and parsed it as an exact statement rather than a standard, commonly used, descriptive term to describe someone who jacked themselves up, then went down a rabbit's hole of determining whether it is possible to put on 25 pounds of muscle in an off season. That's a combination of not having much to talk about yet, and certain people who want to take every statement and parse the words to their exact, pinpoint meaning, even when people rarely converse in casual conversation or informal message board chat with that level of exactitude. A message board post is not a scientific paper, people use words to denote general feelings, emotions, opinions, and ideas, and do not expect everything they say to be rigorously vetted to the tenth decimal place. But around here, sometimes it is.