In that case, I think they’re saying that both CO2 and O2 will be absorbed over time by minerals (as well as CO2 being absorbed by plants), and once CO2 levels drop too much, photosynthesis stops (or slows), which means oxygen stops (or slows) being replenished as well, so oxidization of minerals will also lead to a reduction in oxygen levels.

(And less volcanic activity over time also means less CO2 being added back to the atmosphere, so the chemical weathering process that absorbs both CO2 and O2 into minerals will no longer have an active counterbalancing force from volcanoes.)