That calculation is simplistic as it is based on the current steady-state. The oxygen catastrophe was a massive change away from the previous equilibrium state which first had to deplete all the buffers. That's how we got iron ore veins, by oxidizing most of the iron out of the oceans. And there are other sinks besides iron.
Only if the reservoir is a fifo queue. :) Otherwise there's a distribution, with a tail. But oldest in atmosphere still looks (envelope scribbling...) order Myr? Rather than Gyr. Similar for breath?
