“Non-fungible” more or less means that something is unique and can’t be replaced with something else.
In that sense domain names are true Non-fungible assets because each domain name is truly unique and cannot be replaced with another name. There is only one Google.com (and each country domain name like Google.ca is also unique. It might host some of the same content, but is customized for the particular country that it targets.
The amazing thing about a domain name is that you can host it on a server, and build a business around it. Try doing that with a Crypto Kitty 😉