How can Sundaland fit it?
Most basically, "Beyond the Pillars or Hercules" is to the WEST.
Sundaland is to the far, far East.
Plato wasn't a round-earther.
As I wrote earlier, there are plenty of spots around the globe that probably got incremental or very fast flooding due to the Big Melt. Sundaland was one as were the North Atlantic, Mediterranean, Caribbean, the North Sea and Baltic areas, and lots of others including your favorite (and one of mine), the Black Sea deluge. All of these were within the span of human oral history and Great Flood stories survive in every area of the world.
The Atlantis tale probably has some basis in fact. But I think Plato's story is like that parlor game where one person starts a rumor and passes it on around the room, person to person, until it emerges in wildly embroidered form. Plato was simply the last guy in the chain. I doubt the truth will ever be known unless somebody dredges a hunk of rock out of the ocean somewhere with "ATLANTIS" carved on it.