I think the way to answer that is to read to you the provisions of the Extradition Act. Subsection 32(2), “Exception—Canadian evidence”, provides that you don't need to disclose to the requesting state all of the evidence.
Evidence gathered in Canada must satisfy the rules of evidence under Canadian law
—that would include the Stinchcombe law of disclosure—
in order to be admitted.
That is the catch. The Department of Justice can decide not to put it in. “We won't tell them about it. We're not trying to admit it, so we don't have to disclose it.” That's the problem.