The statement "if you have a phobia of long words, you must tell people that you are Hippopotomstrosesquipedaliophobic" is an example of verbal irony. This type of irony occurs when a phrase or expression is used to convey an opposite meaning or to highlight a contradiction.
In this case, the irony lies in the fact that the term "Hippopotomstrosesquipedaliophobia" is itself an extraordinarily long word, which would likely be uncomfortable or problematic for someone with a phobia of long words. Thus, suggesting that a person with that phobia should announce their condition using such a long word creates a humorous contradiction.