You´re wrong. No one (except you) is claiming Atlantis was 400.000 years old.
Atlantis was pre-flood civilization. That pre-11,12 thousand years ago.
What you are talking about is the last interglacial period (before the one we´re in right now).
There was a pre-flood "Atlantis" but it wasn´t in Greenland. The ice-cap there was just as thick as today, if not thicker.