conflicting airborne fraction definitions
The so-called "airborne fraction" (AF) doesn't represent anything physical. It is just a computed ratio between the observed rate of increase in the amount of CO2 in the atmosphere, and the rate at which human CO2 emissions add CO2 to the atmosphere.
The AF is currently about 50%, meaning that nature is removing CO2 at about half the rate at which humans are adding it. So if we were to suddenly halve human CO2 emissions, the AF would be zero. If we were to more than halve human CO2 emissions the AF would be negative, and the atmospheric CO2 concentration would be decreasing, rather than increasing.
That does mean that "net zero" is unscientific. But it does not affect the conclusion that the sole reason that the atmospheric CO2 concentration is increasing is that the rate of anthropogenic CO2 emissions exceeds the rate of natural CO2 removals.