They are taking the average GDP adjusted for Purchasing Power Parity over 5 years. The problem is I am not sure PPP works for total GDP, it is meant to reflect the differences in consumer income i.e. how far a consumers wages will go. So someone in the US on say $50 000 may only be able to buy the same amount of goods as someone in country x who is on $40 000 because consumer prices are lower.

But it relies on a basket of consumer goods, so I really don't think it works scaling to GDP as a large part of GDP calculations are not in a sample consumer basket and if you exclude things like property (which is insanely expensive in China) you can end up with wild distortions.