Co2 concnetrations are higher in air. Much higher. And the lethal limit for a human at sea level is about 20,000 ppm for one hour
The problem is water resists the uptake of co2 becuase of surface tension. And whats knwon to the science and hobby community as partial pressure.
An airstone would indeed inject tons of co2 and oxygen, but its also riding the 70% nitrogen gas bubble right back out before any of it really dissolves.
Also breaking the surface of water doesnt release as much co2 as an airstone becuase the gas bubbles break the suspension of the co2 molecule easily and absorb co2 and transport it to the surface. Very wastefull