This factors in some feedback loops which most sudies to date have ignored. Note, that it excludes extra warming caused by all the additional water vapour in the air.
For context, studies suggest that the largest extinction event in Earth’s history (so far), the Late Permian Extinction Event, saw temperature rises of around 8°C.
There’s no way they can know that. There won’t be anyone alive to measure after 8 degrees. Checkmate science 🧬 /s