I looked at the chapter, and it seems to be an admonishment not to take "rebirth" literally, but to seek the meaning behind the poetry.
he speaks with equally authority on both earthly birth and the heavenly rebirth (both are equally as litteral), for he knows both (earthly and heavenly), but knows the the pharisies only know birth, so he uses the analogy. to no avail..
John 5:
46 If you believed Moses, you would believe me, for he wrote about me. 47 But since you do not believe what he wrote, how are you going to believe what I say?"