The heavily ironic implication is that they're under NDA, so they can't attest to it, while more or less attesting it. Senator, I cannot confirm or deny that we definitely do this.
This could also be an unofficial-official way for Apple to "leak" that yes, they do this--which is on brand for how Apple handles "rumors" etc.
When the CIA representative says "I can neither confirm nor deny" it generally means the atrocities of which the agency has been accused did, in fact, take place.
When I worked in the civil service we were trained to use that phrase to any query, no matter how innocuous (unless we had permission to give more info).
You may think that not issuing a categorical denial is suspicious, but generally speaking you cannot infer any information from that response. If it was only used when really bad things might have happened, maybe you could infer more.
This could also be an unofficial-official way for Apple to "leak" that yes, they do this--which is on brand for how Apple handles "rumors" etc.