Yeah. Knowing you have the option of getting out - and that earth is just a few metres away - must have a huge impact on the state of mind of the participants. It cannot compare to the feeling of isolation knowing that you are in a tiny tin can separated from home by 50 million miles of howling void and many months or even years of travel time. I'd be very interested to know how they controlled for this in their studies, if at all.

A better simulation would be to dump them on some remote artic ice sheet where they actually can't get back. Or on the moon :)

Edit: in terms of psychological data, I'd imagine that the long flights of some cosmonauts on Mir were a better case study.