What *I* would love to tell him is that the reason for such a disclaimer is because of people like him. Or that he absolutely is right, he doesn't have to sign it, nor stay either. 😎
Ok, seriously, those are your rules and your prerogative based on your reality, not some stranger's. This is a liability disclaimer over the presence of a pool and a very natural requirement. I would take it up with Airbnb before you even considering cancelling a reservation however, give them a chance to deal with it.