My question when I read that paragraph is: they say the addition of a "small amount of gas" has a negligible effect on net radiative flux at the tropopause.
Certainly depends on what they mean by "small", an increase of 320ppm is no "large" increase in my book, especially in comparison to H20 concentrations that vary from nil to 20000ppm as a matter of natural variation day to day and from geographical area to area, and have a direct impact on how much IR is available for absorption by CO2 as well as the problems associated with the CO2 IR extinction distance of <100ft at ground level.
The problem in not what is small in absolute terms, but what is small when dealing with concentrations where IR absorption is at saturation levels as it is with CO2 & the earth's atmosphere.
At those levels, doublings of concentration do not appreciably change the amount of IR that is absorbed as all the available IR at the nominal wavelength is already absorbed. additional concentration cannot cause an additional effect. The ultimate limiter is the amount of IR available in relation to the limiting concentration of the GHG that causes extinction and the exchange of heat to other molecules kinetically.
In the presense of a gas, such as water vapor, whose IR absorption overlaps critical wavelengths, the issue becomes especially important in determining the actual effect of the minority GHG.