[twright0]

> He replies, "I'm Zaphod Beeblebrox!"

This only happens because he was inside a computer simulation of the universe created just for him, so the Vortex shows him he actually is important. Had it happened for real it would have destroyed his mind just like everyone else.

[GMoromisato]

My memory comes from the radio adaptation, in which Zaphod survives because he’s delusionally egotistical.