In questions like this, it's very important to keep in mind the difference between state of knowledge about preference (which corresponds to explicitly endorsed moral principles, such as "slavery bad!"; this clearly changed), and preference itself (which we mostly don't understand, even if our minds define what it is). Since FAI needs to operate according to preference, and not out state of knowledge about preference, any changes in our state of knowledge (moral principles) is irrelevant, except for where they have a chance of reflecting changes in the actual preference.
So the idea is that 21st century American and caveman Gork from 40000 BC probably have very similar preference, because they have very similar cognitive architecture, even though clearly they have different explicitly endorsed moral principles. This property is a "sanity check" on a method of defining preference, not an explicit requirement.
In other words, finding similar preferences in people from different eras is about consistency expected between different maps of the same territory, not about adding a rule that demands consistency from the maps of the territory, even if the changes thus introduced aren't based in fact.
So the idea is that 21st century American and caveman Gork from 40000 BC probably have very similar preference, because they have very similar cognitive architecture
If something like Julian Jaynes' notion of a recent historical origin of consciousness from a prior state of bicameralism is true, we might be in trouble there.
More generally, you need to argue that culture is a negligible part of cognitive architecture; I strongly doubt that is the case.
Taken from some old comments of mine that never did get a satisfactory answer.
1) One of the justifications for CEV was that extrapolating from an American in the 21st century and from Archimedes of Syracuse should give similar results. This seems to assume that change in human values over time is mostly "progress" rather than drift. Do we have any evidence for that, except saying that our modern values are "good" according to themselves, so whatever historical process led to them must have been "progress"?
2) How can anyone sincerely want to build an AI that fulfills anything except their own current, personal volition? If Eliezer wants the the AI to look at humanity and infer its best wishes for the future, why can't he task it with looking at himself and inferring his best idea to fulfill humanity's wishes? Why must this particular thing be spelled out in a document like CEV and not left to the mysterious magic of "intelligence", and what other such things are there?