I will not try to claim I know their motivations. But it's a security/OPSEC rule, and just because it doesn't apply in specific cases doesn't mean it should be thrown out.
And like I said, I believe their main motive is to keep the trust of possible future leakers, in the sense of insisting that they will never officially confirm identity of their sources for any reason.
If they really believed in not releasing the details they wouldn't have hinted at all. They wouldn't have said anything because hinting at the identity is the same as confirming it if it is really the leaker.
The fact that they have but aren't actually confirming it says it all really.
And like I said, I believe their main motive is to keep the trust of possible future leakers, in the sense of insisting that they will never officially confirm identity of their sources for any reason.