If preference is expressed in terms of what you should do, not what's true about the world, new observations never influence preference, so we can fix it at the start and never revise it (which is an important feature for constructing FAI, since you only ever have a hand in its initial construction).

In the situation described by Roko the agent has doubt about its understanding of the very ontology that its values are expressed in. If it were an AI that would effectively mean that we designed it using mathematics that we thought was consistent but turns out to have a flaw. The FAI has self improved to a level where it has a suspicion that the ontology that is used to represent its value system is internally inconsistent and must decide whether to examine the problem further. (So we should have been able to fix it at the start but couldn't because we just weren't smart enough.)