Different sense of the word "authentication"!

What you (and, no doubt, the Apple engineers) are thinking of as authentication is really a signature. Signatures and authenticators aren't the same thing.

Here, we're referring to message authentication, which is the mechanism of using a secret key (usually agreed on at the same time as the secret key for the cipher itself) to apply a "secure checksum" to the ciphertext, to ensure it isn't tampered with.

Trying to use a signature in lieu of an authenticator is what got Apple in trouble here. In addition to ECDSA signatures, those messages also should have used a MAC, or, better still, an encryption mode with a MAC built in, like OCB or EAX or NORX.