The NSA having the information leaked is more beneficial than saying it directly. If at any point something shows it clearly wasn't North Korea, the NSA can claim they have made not official statements and the leak was using bad information. But if such information doesn't come out, then we get what we currently have, which is the public having been informed that North Korea is responsible. What more could the NSA have asked for? Well maybe North Korea directly claiming responsibility, but besides for that.