Sorry about that - I confused your article about Leluh with the related Nan Madol site. Yes, the Leluh burial mounds date back a thousand years. Nan Madol is different.
Carbon dating indicates that the construction of Nan Madol began around 1200 CE, while excavations show that the area may have been occupied as early as 200 BCE. Some probable quarry sites around the island have been identified, but the exact origin of the stones of Nan Madol is yet undetermined. None of the proposed quarry sites exist in Madolenihmw, meaning that the stones must have been transported to their current location. It has been suggested that they might have been floated via raft from the quarry, and a short dive between the island and the quarries shows a trail of dropped stoneS...There's a paper on dating the stones on the Academia website, but I didn't sign in to download it, due to my own laziness. Not much of a mystery here.