When this article was written, the world population was 7.742 billion people. 6 billion people dying would mean more than 75% of the world population.
The shock of this prediction is only slightly blunted when you consider that the world population is currently increasing linearly, and is expected to peak at current growth rates around 10 billion people near 2100. Six billion people dying of climate-related causes is not that crazy over the timescale of 75 years.
His prediction is pretty optimistic compared to the prediction of “Only about 10 per cent of the planet’s population would survive at 4 C.” by professor Kevin Anderson of the U.K.’s Tyndall Centre for Climate Change.