There is quite a lot of extra discussion regarding the 1000-ton rule in the artual report itself (link can ne found in the article). Here are some excerpts:

it is likely more than 300 million (“likely best case”) and less than 3 billion (“likely worst case”) will die as a result of AGW of 2 °C.

A more recent attempt at quantifying future deaths in connection with specific amounts of carbon was published by Bressler [69]. Coining an economically oriented term “mortality cost of carbon”, he claimed that “for every 4434 metric tons of CO2 pumped into the atmosphere beyond the 2020 rate of emissions, one person globally will die prematurely from the increased temperature”. His predictions were confined to deaths from extreme heat when wet-bulb temperature exceeds skin temperature (35 °C).

Some interesting stuff in there.

I would’ve added more but holy shit the mdpi.com mobile website is atrocious to copy stuff from. It keeps throwing me at the end of the entire article, highlighting everything.