This is such a random argument, and you bring it up in every thread about PGP. No, it's not at all the case that authenticated encryption is only meaningful in "stateful, online protocols", nor is it the case that signatures and authentication tags are interchangeable. Even PGP doesn't agree with this weird argument.
This is a category error. Authenticated encryption does not require statefulness. Just look at NaCl's cryptobox.
The state is the authentication here. For stuff that is only sent once you can check the signature and you are done. Otherwise you have to maintain it for the duration of the connection and you are not going to want to sign every single message sent.
In the ways that GnuPG is normally used the practical impact is zero because that is not how a stateless, offline protocol works. The content is authenticated by signing it directly, thus avoiding the extra complexity of a stateful connection oriented authentication scheme. The details here:
* https://articles.59.ca/doku.php?id=pgpfan:authenticated
So should functionality be added here for what would be purely political reasons? Or should the political purpose be spun off into a separate utility?