To answer your question, under international law, the question is not whether the Canadian officials were presented with incontrovertible evidence of torture, the question is whether they were presented with evidence of the real risk of torture and what steps they took in response to this risk to preempt the perpetration of torture.
The U.N. Committee Against Torture, which has responsibility for ensuring state party compliance with the Torture Convention, has stated that the protections under the Torture Convention extend to detainees held by the military forces of a contracting party, regardless of where those forces are situated. The Committee considered this same armed conflict in Afghanistan and found that Denmark violated its non-refoulement obligation when it transferred detainees to the jurisdiction of another state.
He cites this report of the UN Committee Against Torture. The relevant paragraphs therein are as follows.
12. The Committee takes note of the information received that the Danish Special Forces captured 34 men and handed them over to allied forces during a joint military operation in Afghanistan in February-March 2002, in circumstances where allegations later emerged of ill- treatment while the men were in allied forces’ custody. The Committee also notes the State party’s assurance that it undertook a full investigation of the incident reaching the conclusion that it did not violate article 12 of the Third Geneva Convention by handing over the detainees. Finally, the Committee takes note of the State party’s assurances that all detainees were released shortly after their transfer to allied forces’ custody and that none of them were ill-treated while in the said custody.
13. The Committee recalls its constant view (CAT/C/CR/33/3, paras.4 (b)and (d), and 5 (e) and (f) and CAT/C/USA/CO/2, paras. 20 and 21) that article 3 of the Convention and its obligation of non-refoulement applies to a State party’s military forces, wherever situated, where they exercise effective control over an individual. This remains so even if the State party’s forces are subject to operational command of another State. Accordingly, the transfer of a detainee from its custody to the authority of another State is impermissible when the transferring State was or should have been aware of a real risk of torture. (art. 3)
With regard to the transfer of detainees within a State party’s effective custody to the custody of any other State, the State party should ensure that it complies fully with article 3 of the Convention in all circumstances.