“Can a moral claim, which is prescriptive, be true or false in the same sense as a factual claim – which is descriptive?”
We would firstly define “fact”, not in juridical terms, but in metaphysics: “a fact is something that had acquired a structure in our conscience”.
Since Kant that we know that the CAUSE of a phenomena is an idea that only exists in our brains. It is not possible seeing the Cause of a phenomena in any place; the Cause only exists in our thoughts, i.e., the cause must be interpreted. So, a “factual claim” is an interpretation, and if it is universally considered true, then it is also CULTURALLY prescriptive.
The biggest faith that exists is that one of the scientist because it is unconfessable. For a scientist, a science’s “factual claim” (for example, a Law of Physics) is so prescriptive as one the Ten Commandments, because that “factual claim” contributes somehow to the definition of his Weltanschauung (world view and ethics).
Kant called it “The Scandal of Reason”: the fact that we (alone) cannot find out ourselves a conclusive proof for an evidence so simple as the existence of an exterior world. Karl Popper put it in a slightly different way: “the exterior world is a working hypothesis for the sciences of nature”.
It is true that facts are data from experience (for example, in experimental science), but facts are not ONLY data from empiric experience but also from subjective experience which turn out universal by means of intersubjectivity. For example, it is a fact that the logical axioms are not physical (in the sense that they are not constituted of matter).
This means that any moral claim is a fact or a “factual claim”. What we must do is verifying whether a moral fact is rationally interpreted or not (true of false). For any valid moral interpretation (i.e., “moral claim”), VALUES ought to be considered, and values (as well as the logical axioms) are not physical.
Values exist by themselves independently of any utility: for example, the value of Justice exists by itself apart from any utility. The Decalogue contains VALUES: the ethical/moral values ought to be universal, TIMELESS and rationally determined (as it also happens with the Laws of Physics).
So: an empirical “factual claim” can also be prescriptive. And a moral claim can be true or false depending upon a rational interpretation.