Okay, here's the thing: If you have plants with more or less CO2 under otherwise the same conditions you'll have more growth in the plants.
However that's not how the planet works. If you put more CO2 in the atmosphere you'll have for example rising sealevels and larger deserts (and probably more effects that diminish food production). You can't just pick a single effect of increased CO2 in isolation. (Except of course that's what the CO2 coalition does, because they chose to believe fairytales instead of science on climate change.)
ELI5: how will melting ice caps cause rising sea levels? Ice has higher volume than water it's made of, so the effect of ice melting should be sea level falling. At least that's the case of Arctic Sea, which is just this: frozen sea. Antarctic is different, because most of the ice is located on land, above the sea level. But then, shouldn't northern cap melting cancel out melting of the south one?
For floating ice, the mass of the ice is buoyantly supported by displacement of an equal mass of liquid water. As the SI units for mass were originally defined by the density of liquid water at STP, of 1 g/mL, when the ice melts, it supplies exactly as much volume as was previously displaced, so there is no change in level.
As it happens, seawater is denser than pure water, with a density averaging 1.088 g/mL, so if we were just talking about floating ice, 1 kg of floating ice would displace 1 kg of seawater, with a volume of 0.919 L, when the ice (mostly-pure water) melts, it supplies additional volume of 1 L, an increase of 8.8% over the submerged volume of the ice. This mixes with the saltwater to reduce salinity and lower the density, but the overall effect is a slight rise in water level.
Now forget about floating ice. Some ice is not floating. It rests upon surface rock. When it melts, 100% of its volume (eventually) flows down into the ocean. When land-ice melts, it has a far greater impact on sea level. But we only need to worry about land ice that rarely melts due to seasonal variation: ice found mainly in the glaciers and icepack on Greenland and Antarctica. Due to albedo and insulation effects, it is more difficult to melt land-ice than sea-ice, but it is indeed melting at rates unprecedented within the relatively short span of human observations.
(If a land-ice glacier calves an iceberg into the ocean, the immediate effect is to raise sea level by about 86% of the volume of the ice, with additional rise occurring as the ice melts.)
Even if the poles were modeled as just ice cubes floating on water, then melting them would not decrease the sea level, as 1/12th volume of the ice would be above water. So even with this very simple model the South Pole has no way to compensate the north pole’s contribution.
However that's not how the planet works. If you put more CO2 in the atmosphere you'll have for example rising sealevels and larger deserts (and probably more effects that diminish food production). You can't just pick a single effect of increased CO2 in isolation. (Except of course that's what the CO2 coalition does, because they chose to believe fairytales instead of science on climate change.)
This is just the obvious flaw in that argument. If you want the details and all the science there's a chapter on food security in the IPCC report: https://www.ipcc.ch/site/assets/uploads/2018/02/WGIIAR5-Chap... It discusses potential yields as well.