The key is that the AI precommits to building it whether we refuse or not.
The 'it' bogus is referring to is the torture-AI itself. You cannot precommit to things until you exist, no matter your acausal reasoning powers.
If "built" refers to building the AI itself rather than the AI building a torture simulator, then refusing to be blackmailed doesn't prevent the AI from being built. The building of the AI, and the AI's deduction that it should precommit to torture, are two separate events. It is plausible (though not necessarily true) that refusing to be blackmailed acausally prevents the AI from becoming a torture AI, but it cannot prevent the AI from existing at all. How could it?
Todays xkcd
I guess there'll be a fair bit of traffic coming from people looking it up?