''One volcano puts more CO2 in atmosphere than all mankind's efforts for the period of the eruption.''
I tried but couldn't find back up for this assertion either.
Apart from this,
'' human activities emit a Mount St. Helens-sized eruption of CO2 every 2.5 hours ''
https://climate.nasa.gov/faq/42/what-do-volcanoes-have-to-do-with-climate-change/