I'm sure it's an oversimplification of the description of the protocol. For example Apple could use a "base private key" that's just a seed fed to a CSPRNG to g

I'm sure it's an oversimplification of the description of the protocol. For example Apple could use a "base private key" that's just a seed fed to a CSPRNG to generate a series of EC private keys. The client can then rotate through this series of keys, while the "base private key" never changes.