So that question was addressed in the 1950s I believe by one Gilbert Plass who was initially involved in getting data for infrared guidance systems for air-to-air rockets (homing in on jet engines). The simple answer is that those frequencies are not saturated at altitude, i.e. 12 km up or so. Hence this is where CO2 exerts most of its effect. For a full discussion:
> "Plass pursued a thorough set of one-dimensional computations, taking into account the structure of the absorption bands at all layers of the atmosphere. In 1956 he explained clearly, for the first time, that the water vapor absorption lines did not block the quite different CO2 absorption spectrum, adding that there was scarcely any water in the upper atmosphere anyway. He further explained that although some of the CO2 band itself was truly saturated, there were many lines to the side where adding more of the gas would increase the absorption of radiation. His arguments and calculations showed convincingly that adding or subtracting CO2 could seriously affect the radiation balance, layer by layer through the atmosphere, altering the temperature by a degree or more down to ground level."
Here's an additional bit of data that explains this rather oft-repeated old trope about CO2 saturation:
> "The early experiments that sent radiation through gases in a tube, measuring bands of the spectrum at sea-level pressure and temperature, had been misleading. The bands seen at sea level were actually made up of overlapping spectral lines, which in the primitive early instruments had been smeared out into broad bands. Improved physics theory and precise laboratory measurements in the 1940s and after encouraged a new way of looking at the absorption. Scientists were especially struck to find that at low pressure and temperature, each band resolved into a cluster of sharply defined lines, like a picket fence, with gaps between the lines where radiation would get through."
> "Plass pursued a thorough set of one-dimensional computations, taking into account the structure of the absorption bands at all layers of the atmosphere. In 1956 he explained clearly, for the first time, that the water vapor absorption lines did not block the quite different CO2 absorption spectrum, adding that there was scarcely any water in the upper atmosphere anyway. He further explained that although some of the CO2 band itself was truly saturated, there were many lines to the side where adding more of the gas would increase the absorption of radiation. His arguments and calculations showed convincingly that adding or subtracting CO2 could seriously affect the radiation balance, layer by layer through the atmosphere, altering the temperature by a degree or more down to ground level."
https://history.aip.org/climate/Radmath.htm
Here's an additional bit of data that explains this rather oft-repeated old trope about CO2 saturation:
> "The early experiments that sent radiation through gases in a tube, measuring bands of the spectrum at sea-level pressure and temperature, had been misleading. The bands seen at sea level were actually made up of overlapping spectral lines, which in the primitive early instruments had been smeared out into broad bands. Improved physics theory and precise laboratory measurements in the 1940s and after encouraged a new way of looking at the absorption. Scientists were especially struck to find that at low pressure and temperature, each band resolved into a cluster of sharply defined lines, like a picket fence, with gaps between the lines where radiation would get through."