I don't think you understand how jokes work. They are mostly "distortions" of real dialog or events to add incongruous or absurdist elements. Here, Hardy's not uncommon momentary doubt about whether a statement really was obvious, while faintly amusing, is made into a joke by turning the momentary doubt into a 15 minute excursion. People then riff on the joke by turning that excursion into a mathematician presenting an elaborate proof that a statement is "obvious", quite contrary to the meaning of "obvious".