ذَٰلِكُمَا مِمَّا عَلَّمَنِي رَبِّي (This is of the knowledge my Lord has given me). He is saying that this has nothing to do with the art of divination through computation of figures and lines (Um al-jafr and 'am al-raml - not Jafar and Ramal, soothsaying or any other occult act of conjuring). Instead of all that, this is what my Rabb tells me - and I pass on the information. And this was an open miracle which is proof positive of prophethood and a major cause of confidence.