Robert May is discussing the issue of adjustment time versus residence time of CO2 concentrations. A short residence time is likely but the adjustment time is very long. Dyson didn’t discriminate between the two and assumed it was residence time that was important. Thus, May had to write that comment to straighten him out.
As May suggests, that if Dyson was under the impression that a short adjustment time is operable as well, then it is clear how he became a skeptic. As so would I, but I have since learned the slow diffusional aspects of sequestering. Dyson thought that biota was semi-permanent but on the adjustment time scale it is still transitory, as the biota will decay within that period.