The film is challenging you to believe in magic. It's pushing you to the edge where science fiction bottoms out in fantasy, and fantasy asserts itself as the ground of dream, magic, myth, i.e. reality.... The horn of the unicorn is a middle-finger held at all the cynics, materialists, and ironists of postmodernity.
I see the unicorn as a direct challenge to the obligatory profession of disbelief and scorn before the possibility that the world may be grounded in the good, that meaning might override all our attempts to deny its existence, that faith and hope are better aligned with the Real than systematic doubt and despair, that culture is more than a function of memetic replication (as Dawkins argued).