Heck, you glossed over the best parts! You skipped over how Khmer controlled nearly the whole economy of south asia for a while (if anyone wanted to move anything anywhere, they had to go through or stop in the Khmer), thus they spread their culture and goods an absurd distance. Nepal's mixing of Buddism, Sihkism, Tao, and confusionism revolutionize the religions and spread chinese culture to india, indian culture to china, all while remaining culturally distinct, essentially making them a massive religious railway. It's comments like the iceland one that make me sad I'm into history, for it seems few else are.
Except Southeast Asians, especially the Anstronesian subgroup represented by the Chams, Malacca, and Majapahit,..., love to take it to the sea rather than making their trade inland. And trading between these maritime states made up the bulk of Southeast Asian commercial activities. Hence, the Khmers didn't really control the economy of Southeast Asia; in fact I highly doubt if they were ever the largest economy there at all. They might have the largest population but that doesn't necessarily mean they command the largest economy.
Trading between the more land-bound Tai-Kradai and Austro-Asiatic subgroups were pale in comparison with the Austronesian group. To make the matter worse, the Khmers' two immediate neighbors are Vietnam and Siam. One only trade with East Asia (via the sea) for most of its history, and the other pretty much declared war on the Khmers once it was formed.