Im looking for more of an official document/announcement. I wasn't able to find anything of the sort in the provided link.
As far as I know, the US has not publicly admitted it created Stuxnet. But at this point both security researchers and journalists have concluded that it was created by the US and Israel, so it's essentially a well known non-secret.
The only reason i ask is because publically concluded and legal fact are far from the same.
For NATO to come out and accuse the US of illegal acts you would think they would need more then hearsay to stand on.
Oddly enough, in international law you normally accuse them if you only have hearsay. If you have proof, then normally you are beyond any sort of international intervention either because you will let it slide anyway or you already plan to strike back regardless of international favor.