Someone who knows the truth you are trying to get from them is very likely relatively early on to try the actual truth to get out of the torture.

If you are smart enough to realize that torturing someone who doesn't know the truth, and have the resources to check multiple hypotheses presented by the tortured, and place enough value on getting the truth, torture on the whole is still really effective.

I think it is just a hopeful belief to say "torture does not work."