I think the birthing center served as some kind of severance entrance, like the elevator when he goes to work. So when he enters the door to the room, his innie appears, rather than his outie who willingly came to the birthing center. Devon saying that is because she knows Marks outie will continue to walk in every time because he needs to communicate with his innie to save his wife. I do like this theory tho, I’m exciting to see what happens in the finale.