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.
If you want to absorb 300g/​day of CO2 you need the plants to grow by ~300g/​day. Which is a lot!
(It’s only rough: 300g of CO2 is 12/​(12+2*18) carbon, but when absorbing carbon plants also take in water with goes the other way)
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.