Difference between Symmetric and Asymmetric. Is this answer correct, based on a written test?
I don't believe he is answering the right question.  You essentially asked "why are public keys so much larger than symmetric keys", and after his first sentence (which started to address the question, but was a bit vague), he tried to answer the distinct question "why are public key operations so much slower" (not that he got the details of that correct; his last sentence appeared to imply that public key operations take an exponential amount of time).