Well, to assert that "an extrapolated version of mankind's combined values can be created" doesn't really assert much, in and of itself... just that some algorithm can be implemented that takes mankind's values as input and generates a set of values as output. It seems pretty likely that a large number of such algorithms exist.

Of course, what CEV proponents want to say, additionally, is that some of these algorithms are such that their output is guaranteed to be something that humans ought to endorse. (Which is not to say that humans actually would endorse it.)

It's not even clear to me that moral realists should believe this. That is, even if I posit that objective moral facts exist, it doesn't follow that they can be derived from any algorithm applied to the contents of human minds.

But I agree with you that it's still less clear why moral anti-realists should believe it.