Pretty much 100% sure that the game is, in fact, a loop. Reasoning: First, the body of the first dead crew member looks like you (just...more dead). Second, the room where the gate slams behind you has a bookcase (top right corner) that notes that there's the word "Remember" carved into it, with (a) tally mark on it. I believe that every time you complete a loop, you get another tally mark, but I'm not going to be the guy that plays the game over 100 times trying to find a secret.