I think (though, again, I only read it quickly) the paper includes estimates for both carbon sequestered and biomass growth for the plants.
I believe the plant which they used as the reference for that rough figure above increased in biomass by 132.5g but sequestered 56.4g carbon over several weeks, and the 0.8g carbon fixed per day comes from that latter figure.
I think (though, again, I only read it quickly) the paper includes estimates for both carbon sequestered and biomass growth for the plants.
I believe the plant which they used as the reference for that rough figure above increased in biomass by 132.5g but sequestered 56.4g carbon over several weeks, and the 0.8g carbon fixed per day comes from that latter figure.