So the authors of that paper have a difference of opinion with the IPCC (and indeed, nearly every climate scientist on the planet).
That's not actually an issue. They're written their paper and now it's up to them to defend it from the inevitable slings and arrows coming from the scientific community. That's the way it works.
Having said that I don't particularly agree with their conclusions. Although if you actually read their paper and their conclusions it seems as though they're not too sure of their position either:
To answer the question of whether the additionally emitted CO2 in the atmosphere is indeed a greenhouse gas, it would be necessary, among other things, to conduct additional research for a radiation source with a temperature similar to Earth's surface temperature and measure the absorption of thermal radiation in a mixture of CO2 and air at different temperatures and pressures, as is the case in Earth's atmosphere at various altitudes.
