I’m not sure that the lack of landscape Face ID would prevent someone from pointing your phone at your face while you were in bed. If your face was sideways and your phone was sideways, then it would appear to be a portrait orientation to the phone and it would unlock. This limitation prevents the phone from recognizing a face when the face is 90 degrees off from the phone. But there’s nothing to stop someone from rotating the phone 90 degrees to match the orientation that your face is in.
However, all that said, Face ID requires your eyes to be open to authenticate. So no one would be able to use your face while you were sleeping anyway.