@Doctor Syntax
I believe that's the point - that they were ordered to bring the device in, which would of course identify it and thus be trumping the NDA, as you say.
The point is that instead of doing that, they offered a deal, which the defence accepted.
In other words, the police let a criminal (not a major one, it must be said) go because they deemed it more important to protect their own misconduct (and possible breach of constitutional rights) than to prosecute him.
Personally, I think that when there is a plausible suggestion that the police have breached the constitution, there should be a mandatory investigation by the courts.
Biting the hand that feeds IT
