That's not entirely accurate. DN3D required a large room behind the mirror (I'm saying this from the perspective of editing levels), but that room was completely invisible and did not have to look like the mirrored room at all. The mirror actually "mirrored" the scene. IIRC, if you disable collisions then you could walk through the mirror into that big room, which looked quite boring.

I always assumed that the "big room" was to avoid the culling algorithms / potentially-visible-set tripping over the mirror.