The agricultural systems around the big temple complexes suggest that over a million people lived in the area.

http://ki-media.blogspot.com/2007/08/metropolis-angkor-world...

Europe was in a fairly decentralized state but still there were many chinese kingdoms quite a bit bigger.

They must mean larger within some other context...perhaps temple area covered? Angor Watt is sprawling and flat. I could def see the surface areas of these complexes being the "mostest"

Looking at Wikipedia it seems wrong. It had the Khmer Empire at 4m population 1.2m sq km, and the Song Dynasty in China at 118m population, 2+m sq km at roughly the same period.

It does say however that Angkor was the largest pre-industrial city in the world, with an urban sprawl of at least 1,000 square kilometres.