Increased concentrations of CO2 may still be responsible for some warming, but that does not change that fundamental fact.
Water vapor, carbon dioxide, methane, and other trace gases in Earth's atmosphere absorb the longer wavelengths of outgoing infrared radiation from Earth's surface.
Methane absorbs more infrared (per fixed amount) than C02, sure, but overall there's less methane (currently) and so the current volume of C02 in toto absorbs more infrared than the current methane volume.
Also relavent to the discussions is the relative rates of change .. the C02 concentration in the atmosphere has been less than 1900 levels for millions of years and has only recently in the past century climbed upwards in concentration by significant amounts .. leading to increases in water vapor and mthane levels.