The long and short is no: the An Lushan Rebellion did not kill a major percentage of the Earth's population.

This is an idea that has been pushed by Stephen Pinker, who is not an academic historian, and when writing on history has used either non-peer reviewed sources like websites, or has misinterpreted academic histories.

Here is an answer I gave on his figures for the Mongol conquests. Similar processes apply to his An Lushan estimates, ie confusing a decline in registered households by government census with an absolute population decline, when if anything it actually shows a more limited census.

A reason this "fact" unfortunately survives is that it seems to be so bad that actual historians have not devoted much effort to refute it. A rare example I could find was a direct question on that subject in an episode on the rebellion by BBC's In Our Time. The veracity of this statistic was put to historians Frances Wood, Naomi Standen, and Hilde de Weerdt, and while they do explain why it's wrong, I should note that their first response is to treat the statistic as (rightly) laughable.