The key to this narrower but much higher estimate can be found in the real world observations around the role of water vapour in cloud formation. (sciencedaily.com)
Positive water vapour feedback in climate models confirmed by satellite observations. (nature.com)
If it were possible to leave the clouds but remove all other water vapour from the atmosphere, only about 40 % less infrared of all frequencies would be absorbed. (newscientist.com)