The distinction they're drawing is between ignorance of the law (e.g: you take someone's phone because you didn't know stealing was illegal) and lack of knowledge/criminal intent relating to the act you committed (e.g: you take someone's phone because someone sold you a stolen phone and you were unaware).
Former is what "ignorance of the law is not a valid defence" applies to. Latter can be valid, and particularly in this case I'd find it hard to imagine that those affected by the dusting attack would be found guilty of violating sanctions.
