I don't think that Watts himself is a scientist, though he is reasonable well versed in the science due to immersion of nothing else.  Most of the stuff on his site (including the article quoted) seem to be written by others in what he often calls 'guest posts.'

Someone (I think Dr. Salby) made mention of a related concept that even pure CO2, for instance, approaches a point in terms of concentrations where it has no more effect.  The corresponding analogy would be if one is already behind an armor plate, more armor plates are not going to have much more of an effect.  This makes some sense qualitatively but I've not run across a quantitative exploration of this principle in the various plausible atmospheric conditions that we might see.

I just scanned around the 2015 London Salby vid.  Around the one hour mark he does some exploration of the impacts of burning every bit of fossil fuels that we can find, and he talks a little about the opacity curves calculated for increased CO2 concentrations.  Interesting stuff (to me.)  There seems to be very little love lost for Salby even among some of the most prolific of Watts's guest posters.  I personally appreciate the guy's presentations though.  It seems to me that his constructs line up with observation better than most.  He seems to be willing to throw out ALL of the 'science' produced under IIPC 'scientists.'  (He also seems to be one pissed-off MF'r.)  I wonder if some people are worried that such a giant disposal of current climate understandings might be a pill to big for almost anyone to swallow and thus he is as big a threat to the 'deniers' as he is to the 'warmanistas.'