The paper states: The climate change debate has revived and reinforced the belief, widespread among climate skeptics, that volcanoes emit more CO2 than human activities [Gerlach, 2010; Plimer, 2009] and
The nearly 9-hour duration of both the Mount St. Helens and Pinatubo paroxysms gives average CO2 emission rates of about 0.001 and 0.006 gigaton per hour, respectively. Intriguingly, the anthropogenic CO2 emission rate of 35 gigatons per year— equivalent to 0.004 gigaton per hour—is similar. So, for a few hours during paroxysms, individual volcanoes may emit about as much or more CO2 than human activities.
It is true there is much bullshit published in science papers in the name of science. Some things that come to mind are "increased extreme weather" which upon analysis is just weather, nothing special with plenty of historical precedent. But the carbon dioxide source debate is completed for now in science. If you look at the people that cite the 2011 paper, you get papers like this 2012 paper: "Another claim concedes the rise but asserts that its cause is volcanic. For example, Plimer (2009) declares that volcanoes emit far more CO2 than humans. Corollaries of this claim state that emissions from one large volcano, or from seafloor vents, dwarf various human sources. But as Terry Gerlach observes in Eos, Trans. Amer. Geophys. Union, such claims flatly contradict the science (Gerlach 2011)." https://journals.ametsoc.org/doi/pdf/10.1175/WCAS-D-12-00008.1
So while it is true that Plimer's claim is suppressed from google search results, it happens to be wrong. Plimer's book was not scientific, yet it was reviewed and considered by some scientists and found to be incorrect on the Pinatubo claim.