Pure muscle, yes. You are right that it is likely impossible for any human to gain 25lbs of pure muscle. We have discussed this at length elsewhere in this comment chain.
That's not what I said though. I said I gained 25lbs of WEIGHT. There is definitely water and fat in there too dude. Just because I stayed visibly lean doesn't mean it's all muscle nor do I state as much. The link you've cited does not contradict what I have said here.
I gained 25lbs and didn't lose my abs in the mirror.
It's not impossible. I made a similar jump in High school when I went from cross country to throwing discus. From the beginning of winter break (weight 177lbs) I added weight and reached 205lbs. This was over the course of a roughly 6 week period of time too.