I'm sure poor countries would love to turn their rice fields into soccer fields and feed themselves with more nutritious food, but they need to farm cheap calories to feed themselves with the amount of money they have.
Things Sri Lanka can do:
1. eat more rice, all 1.2 mil tons of it, for the same amount of minerals (this would lead to obesity).
2. plant and eat 1 million tons of rice per year, leaving empty 1/6 of the previous fields free for other use. In those 1/6 of the fields, they can now grow carrots and mushrooms and lentils and other highly nutritious stuff. It might even leave them with extra land for football fields.
How is this an issue? I swear: sometimes people want to find a negative even in the most positive outcomes.
Don't forget to factor in the cognitive impairment from higher CO2 as well - even if CO2 magically made the food both more nutrient-dense and with higher growth rate (instead of higher growth rate with lower nutrient-density) the overall health effect on humans would still be presumably a massive net negative due to the cognitive function decline.
I just detailed on how that will happen, no need to use magic. Increased CO2 will be better for plants worldwide (including deserts) and for agriculture, that's maybe the one positive thing I can find about global warming.