I don't know that I would take the Time's reporting on the investigation to be the end-all-be-all of what evidence there is. Like you say, accusing a Nation State probably requires a decent amount of evidence, and I doubt they would release it all to the NY Times. The article even says

> It is not clear how the United States came to its determination that the North Korean regime played a central role in the Sony attacks

To me, that sounds like they aren't privy to the reasoning used.