To add to your idealization of adding muscle only with no fat, imagine the muscle stays the same size but just gets denser as it gets stronger (so that the levers behave the same). Then I would think the ratio only improves and a pull-up becomes an even more sub-maximal lift as bodymass increases.
However the ratio of potential force generation vs gravitation force is not the only thing that matters to the climber. Each pull-up becomes increasingly sub-maximal with bodyweight increase, but it also takes increasingly more energy to accomplish, since the climber has to pull up the increased mass each time. The heavier the climber is the more energy is required for each pull-up, regardless of how difficult/submaximal each pull-up is. Muscles store a finite amount of energy that is replenished at a finite rate.