there's a lot of unmeasured assumptions and if you read what is described to the public its usually scientifically wrong. usually it's one of:

- methane has a higher absorption than CO2

incorrect. CO2 has a dipole moment amd c-infinity-v symmetry so it absorbs way more

- methane has higher absorption in open windows of IR frequencies

also incorrect. the water band don't overlap with CO2

- methane has a longer atmospheric half-life

incorrect. you can look up the numbers on this. i believe it was believed to have a longer half life a few decades ago but detailed isotopic studies have disproved it?

you have to dig really deep to figure out that there is I think? an estimated self-shading effect of CO2 that changes the marginal absorbance of a single molecule. but this assumes a uniform distribution of CO2 in the atmosphere and no scattering. anyways i think this is not spoken of because it also reminds that the effect of Co2 is logarthmic (A = log(T))