I think it’s as simple as biomass produced vs biomass consumed, and I agree a 4x4 garden is not going to fully replace O2 for two sleeping adults. I’ve experimented with building tower setups which would allow 3-5 times the biomass growth in the same space, but it’s a real pain tending to a garden at eye level, and introduces all kinds of new catastrophic failure possibilities when you have water above high power lights.

Nurses used to take plants out of hospital wards overnight because photosynthesis (input CO2 output oxygen) only happens in the daytime, leaving just respiration (input oxygen and output CO2) at night.