That's an interesting study but I fail to see that it supports your claim. There's no mention of C4 fixation in the article, nor any claim that CO2 fixation is

That's an interesting study but I fail to see that it supports your claim. There's no mention of C4 fixation in the article, nor any claim that CO2 fixation is a significant limiter for agriculture. The article claims 30% increase in general (not just agricultural), but also does not attribute all that simply to more available CO2. Your own claim is self-contradicting as you use arid environments as the basis for your claim. Agricultural environments where plants are irrigated are not arid, by definition.