It's difficult to imagine an authentication system that doesn't have some kind of centralized mechanism for making sure identities aren't duplicated. In this case, delegating that to a combination of two existing technologies (DNS for the domain, then email for the username) that are open, well understood and easy to implement seems appropriate.
What's the alternative? Using some kind of pseudo-GUID and then maybe a derivative of the Paxos distributed consensus algorithm to decide if it is to be trusted? I'd imagine there would be a good number of PhDs in that approach before a system like that would be close to being ready for actual implementation.
This system seems just about as decentralized as is practical. If you disagree I'd be very happy to hear alternatives.