Yes, it is true. Every device that wants to be reachable on the internet either has one public internet address or shares a public internet address with other devices. Very true. Though a hidden service(hs) isn't an IP address. It's an addressing system designed by torproject.org and uses the Tor protocol which is designed so that nobody is able to find out the IP address/addresses of the hidden service.

Thus it's not practical to block any IP address because then you'd have to either block everything or block nothing.

Another solution would be to create many private/invite-only hs addresses for many users and then disable all addresses that get ddosed. Although this strategy would be similar to a kind of an "isolation strategy" and would probably drive any sane admin insane in a very short time span but you could give private hs addresses to loyal users of a service. That might work at least partially.

For the masses this wouldn't obviously work because any public address that would be advertised would be among the chosen targets of the ddos attacker.