I think they mean from a user perspective. As a user I can create an account with a provider(be it google or myself), but I can't migrate that account without leaving a forwarding address at the original address.

The alternative would be storing the user data on a distributed data structure(like a pki network or distributed hash table) so that it's not tied to a single provider.

Because if you host your own email server, the email address has a domain or possibly IP address to point at your server. But getting domains, or getting IP addresses are both centralized.

If you gave your email address to someone, you cannot change it on their devices. Your domain registrar with DNS for the domain case, or your ISP for the IP case, can stop email deliveries to you or redirect them somewhere else.

With a decentralized email, people email "you". Not "your username" on Google's service, or your username on a domain you rent from others, and not your username on an IP you rent from a centralized authority.