No, you don't know for a 'fact' that niqaab is not waajib; you just think you know. These are two different things. He is not ignorant for believing that niqaab is waajib. The great companion of the Messenger , Ibn Abbas believed niqaab to be waajib for example; would you dare call him ignorant? Does he know more or do you? Please search for this topic and look at the evidences presented. The ruling of Hajj and Umrah is different. This is explained in all of our books, from centuries ago. You should probably study some of them