If I accept that the NSA is competent in their intelligence gathering capacity. An organization does not get away with what they did for years and keep it secret for any period of time by being incompetent. That said, AFAIK the NSA has not stood behind this information, the FBI has.
If the NSA were able to identify with some certainty who the attackers were and it was another nation, and they forewent actually using this as an example of a real tangible good that comes from the intelligence gathering apparatus they've put in place after all the flak they've gotten, to me that colors the information somewhat. If they don't want to stand behind the assessment, it could be entirely political (none of the controversial programs helped, so don't fuel the fire of people asserting they aren't worthwhile) or it could be operational, as in they aren't as confident as the FBI is professing. I'm sure there are many other possible reasons as well, but I don't think it simply boils to competence or incompetence.
If the NSA were able to identify with some certainty who the attackers were and it was another nation, and they forewent actually using this as an example of a real tangible good that comes from the intelligence gathering apparatus they've put in place after all the flak they've gotten, to me that colors the information somewhat. If they don't want to stand behind the assessment, it could be entirely political (none of the controversial programs helped, so don't fuel the fire of people asserting they aren't worthwhile) or it could be operational, as in they aren't as confident as the FBI is professing. I'm sure there are many other possible reasons as well, but I don't think it simply boils to competence or incompetence.