This appears to be a study of physics, heat transfer and heat retention due to 'instant' change in CO2.  It did not look at feedback loops, like melting of glaciers or polar ice causing more heat retention, it is not a "climate" model, but a temperature model. Doom is still quite possible, with feedback loops like that, release of methane from melting permafrost**, release of coastal plain frozen methane, and 'doom' from sea level rise flooding coastal cities, infrastructure.

 The authors wanted to know what would happen, in essence, if the amount of CO2 in Earth’s atmosphere doubled instantaneously. .. climate change could launch unpleasant feedback loops, causing more carbon dioxide to enter the atmosphere. For instance, if runaway wildfires were to torch millions of square miles of forest, then the carbon once stored in those trees would enter the atmosphere. But climate sensitivity isn’t meant to measure that feedback.

** Right now, the Earth's atmosphere contains about 850 gigatons of carbon. (A gigaton is one billion tons—about the weight of one hundred thousand school buses). We estimate that there are about 1,400 gigatons of carbon frozen in permafrost