No, you don't need a single authentication provider, you can use more or less what you want/need and there is nothing for it mandatory in the protocol.
What currently is kinda centralized is an identity matching service, but even that is completly optional and will bec replaced by something more federated at some point.
What currently is kinda centralized is an identity matching service, but even that is completly optional and will bec replaced by something more federated at some point.