the number of volcanos erupting in some interval (say 10 years) is large enough that you can roughly treat it as a constant source of CO2, but at the same time

the number of volcanos erupting in some interval (say 10 years) is large enough that you can roughly treat it as a constant source of CO2, but at the same time the volcano is emitting igneous rock (i.e. the magma with CO2 removed), which will reabsorb the CO2 over a very long timespan... so yes volcanos emit large amounts of CO2, and commensurate amounts of CO2 absorbing igneous rock, in geological timescales they cancel