That's interesting and all but-- I admit I haven't read the source-- I was under the impression that the actual text was fairly clear about Atlantis being a fictional political allegory.
With that in mind, they would not have placed a high priority on clearly stating what was totally real and what was totally unreal. Atlantis could have been a real city in accordance with the theory described by OP, but Plato might have adopted that truth for the purpose of crafting a political allegory that would be, in his eyes, even MORE true than some mundane fact like "Atlantis actually exists." So don't be too quick to dismiss it as false just because Plato meant it as a political allegory. The ancient Greeks didn't care as much about the distinctions we cling to in the modern world.