Oh, I'm pretty lightweight / lanky (hover around 165lb, BMI around 20-21). I didn't see any significant weight change at the time, perhaps I transferred muscle if that's a thing since I was doing less general muscle building at that point - just pushups and the casual pull up routine, running (sprints) to blow off steam.
I can see weight loss being a significant factor for heavier people, esp. those that are heavy and strong. I am definitely neither (typical things like bench press / squat etc I used pretty light weights).
Eh, you can also improve your lifts by just not being exhausted every time you go to the gym. With lifting, more is not always more. Fatigue and failure to properly recover is a fairly common reason for plateaus, especially with full body pulls.
you either lost significant weight, or added significant muscle .. no other way .. the end result here is more significant than how you reached it
for pull up, loosing weight is usually far more important than adding muscle, so i am leaning toward you lost weight