That is correct. Several conventions explicitly prohibit countries from transferring detainees. But the prohibition on torture is considered a jus cogens norm. It is the most fundamental principle of international law. It's so serious that states are required to seek out people who have perpetrated torture and prosecute them. So international courts and commentators have read into every other treaty that doesn't explicitly deal with the obligation of non-refoulement the obligation that states not transfer people to face torture.
On May 5th, 2010. See this statement in context.