Plants don't sequester the carbon, like you said. But on the other hand, the extra CO2 is causing a bigger plant mass growth. So called greening of the deserts (but not just deserts). So basically the living biosphere is growing and absorbing CO2 as it does, because it can.

The plants evolved when there was more CO2 around, 200-300 is very close to starvation, they thrive in 1000+ environments.

Total Earth biomass is 550 gigatons, much larger than 36 which is what we emit every year. But obviously it cannot keep up...since the year 1750 we emitted 1700 gigatons, so more than 3 times the weight of the biosphere. That being said, I can see how the biosphere can grow by 2% with all the extra CO2 and go from 550 to 560, 570 and so on.