The problem with early civilization type theories though is this. Advanced tech requires specialized materials. That means mining, transportation, and processing. We should find roads, and mines, and slag piles, and processing plants.
It's pretty hard to hide roads, especially the types of roads you need in the mountains where you mine materials.
On the flip side, though, they would have had access to much richer ore bodies than we have today.
They're underwater on Sundaland. The prime area for tin mining in that region is on the Sunda Shelf. The oldest bronze manufacturing/smelting site ever discovered is in Thailand.