Mangum's probation order requires Mangum to submit to drug-testing when requested by the police or the court. The results of that test could conceivably be held away from the Duke case for that reason. Whether it was positive or negative, Nifong would definitely want to keep it back. Only if it was positive for a date-rape drug would he want to use it.
You are probably right about the source of the toxicology examination.
However, the fact that the examination was done under the probation order should not relieve Nifong of his ethical obligation to disclose exculpatory or impeachment evidence.
I chalk this up as one more incident of ethical misconduct by Nifong.