I am somehow not convinced that biotic factors are solely responsible for creating 21% of atmosphere (around 40 million moles of oxygen). There may be additional issues here. Since cyanobacteria produced oxygen, it should have developed antioxidant machineries. By the time cyanobacteria flourished, some dependent heterotrophs must have evolved and they also would need antioxidants, as they are expected to thrive in close proximity to the autotrophs. There is a likelihood that aerobic respiration co-evolved with biotic oxygenesis. This would have further delayed the build up of oxygen in the atmosphere.
Does anyone have an idea of what is the photosynthetic oxygen production rate per gram biomass of cyanobacteria? Since photosynthesis must have happened mostly at the surface of the water (limnetic zone), the total biomass would be restricted. Then it might be possible to calculate the time required to achieve the oxygen levels of 21%.
This measure can indicate whether the current theory is plausible or not.