The madman is criticizing tradition. He criticizes why the Chinese have always accepted this same isolationist tradition for thousands of years, and they never modernize and/or reach out to other countries. Xun is not really discussing 'cannibalism,' but criticizing the overpowering, burdensome isolationist tradition of China. It is cannibalistic in the sense that you are "eating" someone else by holding them back from thinking independently and avoiding any modern and/or foreign ideas.
Another way to think about it is if we are all cannibals in a way. Are we all open-minded to everyone else's ideas? The madman is obviously criticizing the obsolete tradition of China for suffocating him and the Chinese people by not allowing them to share ideas openly with other cultures. But in the same train of thought, the madman is being "cannibalistic" because he is all for openness and being closed-minded about China being an isolationistic society. Is there a true compromise of sharing all beliefs and being open to new ideas without being critical, condescending, and condemning of another opposing belief? I believe it is not possible, since many views oppose each other very stringently. There is no way to accept two opposing views without ever being bias toward one side. If someone says they are open, then really they probably are just indecisive and really have no stand on the subject any way.