It reminds me of the souterrains or earth houses you find in the north east of Scotland, just north of Dundee. I visited the Carlungie eart house a number of years ago when I was in Scotland. It was a bit bizarre - it's the only historic site I have visited where you have to walk along a pathway through a potato field to get to it.
Looks primordial, but I guess the built upon those stone structures so their dwellings were above and below ground level?
I don't know about the site in France but the general consensus on the Scottish examples is that they were originally below the ground but the top half of them has long since vanished. Some of them were associated with groups of above ground dwellings.
How nice it would have been to live there. Wonder what the 'perishable' upper levels would have looked like? Surely a neolithic fishing village where the fishing would have been really productive in those waters.
My guess is that the lowering of their habitations is to be clear of wind drafts maybe?
Chances are that the people (and Age) that produced the earlier migrations to Australia were also knocking about in Europe. I wouldn't be the least surprised.