That's a cool study. CO2 fertilization is a real phenomenon (with some caveats, which I pointed out some of), but I stand by the characterization that it is misleading to use this to suggest that we shouldn't be concerned about (or consider reversing) anthropogenic CO2 emissions, or to suggest that plants are maladapted and struggling without "CO2 concentrations at least 5-10x the current levels". Plants, algae, and cyanobacteria have had plenty of time to evolve carbon concentrating mechanisms [1] to help offset low atmospheric CO2.
My apologies if that isn't what you were doing.