Anecdote: When I artificially inflated the CO2 in my aquarium, my plants grew like BANANAS. Something like 4x the speed, and about 2x as big leaves, etc. It was quite something amazing.
And it's been proven to be true many times.
I guess what the study is trying to address is the positive aspect of CO2 rising enough to counter the negative aspect of CO2 rising to the plant biomass.