Not sure if this is a spoiler, it’s been a while since I read those books, but if memory serves the only reason Zaphod survived the TPV was because he was temporarily the inhabitant of a pocket universe specifically designed to trick him, and naturally for this universe’s version of the TPV he was the most important being in it, and in telling him so the pocket-universe TPV just confirmed ZB’s own view of himself, leaving him unharmed and a little extra smug. At some further point in the plot this fact is revealed, not sure if it’s the same book, but I remember it as a hilarious deflationary moment for the character.