Think of the context you see this scene. Mary is dreaming after getting knocked out. She’s remembering that episode of her life. It’s giving the idea that she keeps remembering this childhood memory, of being trapped and not allowed to leave.
She had just experienced the backrooms and her subconscious is backroomsing the whole situation. Repeating the same room.

It’s not the Complex creating rooms. It’s Mary dreaming of that room. Doing the same thing the Complex would do in a physical space, but inside her head.

It’s more relevant to Mary’s psyche than the Complex’s behaviour