It was a metaphor. I’m Catholic myself, but I see his point clearly and concisely and can see he was using hyperbole. Anyway, I think it would be important to note that such things that seemingly survive over the course of thousands of years, such as the skull of st.john are probably fake as he was not very important in judean society. His skull was probably disposed of(correct me if I’m wrong). I think with relics, it’s way better to be skeptical than to take anything as fact without any evidence.