It’s been known for quite a while that the increasing amount of atmospheric CO2 is much greater than can be explained by anthropogenic CO2, especially since the increase started before the beginning of the Industrial Revolution.
Something not mentioned here is the oceans acting as a CO2 sink, taking CO2 from the air. If these volcanoes are releasing to the oceans some great amount of CO2 then that must, I would think, much alter their equations. I think that's suggested in this release and probably appears in the study.