What this is overlooking is the temperature dependence of photosynthesis. C3 efficiency starts dropping around 25C, and shuts down entirely around 45C. C4 photosynthesis works at a bit higher temperature, peaking around 35C, but even it shuts down around 55C:
https://kuensting.org/school/bb/cell...osynthesis.htm
So you put enough CO2 into the air to raise temperatures into this range (even if only in the tropics), and then goes a chunk of the biosphere. And of course it's a positive feedback: fewer plants recycling CO2 means more goes into the atmosphere, which means more temperature increase, which means even less photosynthesis...