I don't think he found that at all. Their guess about coal reserves is just a guess though perhaps more educated than some. They note that the energy value of U.S. coal may already have peaked, but that's not the same as a CO2 peak. They surmise that Russia will be the last great coal exporter, but they don't know how much coal Russia will be able to mine and produce. No one can yet determine with any kind of accuracy when the coal peak will come.

Coal is the industrial energy source of both first and last resort. The re-electrification of transportation will preserve industrial civilization for a while. Coal will not be able to cover both baseline electrical demand plus that project. Nuclear, wind and concentrated solar power will play a role. The real energy collapse comes not when oil peaks but when coal peaks.

The papers on energy reserves and how much CO2 they would produce and, thus, their effect on climate were all done prior to Hansen. et al.'s new work on sensitivity, so those papers need to be reconsidered through that new constraint.

Cheers