Im going by the ansem report clearly if dreams were such a big part wouldnt ansem have listed as an important part?
If the unbirths were really made up of dreams, then Ansem wouldn't have known because he studies hearts. It doesn't seem like anyone in the present KH timeline knew about the unbirths, so it wouldn't make sense for people to be specifically studying them and making the connection between dreams and something that doesn't exist anymore.