My guess, with AT&T being an enormous and ancient corporation long past it's glory days, is that internally its a stale mess serviced by people unwilling to dive into it.

The breach did happen, but things under the hood are so bad that they have no idea it happened. The layers of incompetence and don't-give-a-fuck completely obscure the evidence. The IT team, staffed mostly by young green cards who weren't even in this country 3 years ago, stare blankly at AT&T's internal auditing system developed in the 90's with a long dead and strictly proprietary language. Doesn't matter because the system didn't even catch the breach anyway. As you move up the chain, people just get more divorced from reality as they live in the delusion of AT&T still being a forefront technology company. So of course the breach didn't happen to them.

At least that it my theory.