Joyce doubts whether myths like this should be counted as items of belief. Instead, he suggests, they are (more like) practices (see p. 236). Here, I think, he is closer to Wittgenstein than he is for most of the book. In the penultimate paragraph of the book he writes:
When appealing to myths is a well-defined practice -- as in the Trobriand lili'u, for example [Malinowski's example of narratives of the third class] -- then all participants are aware that they are "special" ("sacred," is Malinowski's preferred term. All parties know that the telling of a lili'u is a serious and distinct kind of appeal, and they are not likely to mistake it for the telling of a fairy tale or for the communication of "ordinary" information. But if a culture diversifies and fragments, or mis-comprehends its own traditions, or for whatever reasons impoverishes its own categories of assent to the extent that the only recognized kind of important positive attitude is belief, then this understanding may be lost. Fictionalism is predicated on the assumption that encouraging a habit of false belief has inevitable deleterious consequences. Its fragility is that a fiction that is presented as being of central practical weight, as something demanding allegiance, is likely to be read by the careless as something demanding belief. (p. 240)Shades of MacIntyre here. I wonder how much our culture is in the kind of mess suggested here and, if so, how much philosophers are to blame. But I wonder also why Joyce excludes myths from the class of things that can genuinely be believed or, put another way, why he takes such a narrow view of belief. Surely some religious belief lies, as it were, between what he would count as belief and the attitude that Malinowski describes toward myths. That is, some believers in the Day of Judgment insist that they believe this in the ordinary way that I believe that the sun will go down this evening, but others regard that as a misunderstanding (perhaps even a sinful one) of such matters. Of these others, some might say that their beliefs are very much like Malinowski's myths, but others will insist that that too is a mis-characterization. And I think one could say the same, more or less, about ethical judgments. Wittgenstein's distinction in the Lecture on Ethics between relative value (value in the ordinary, down-to-earth sense) and absolute value (value of an ultimately mysterious, other kind) would be accepted by many people, I think. But absolute value is not a myth in the way that an atheist's talk of hell might be. For instance, a non-religious person, i.e. someone who freely accepts that they do not really believe in hell, the sacred, etc., might still find that they want to refer to hell, the sacred, and so on in order to express their beliefs. (Ronald Dworkin talks about the sacred in Life's Dominion, e.g., as does Morrissey in "Suffer Little Children," and I don't think either one of them is religious.) Such people might say that their words are not true in the ordinary sense but do express a higher truth, or something of the sort. This would be at least close to using words in what Wittgenstein calls a secondary sense.
If I say that murder is absolutely wrong, or a non-fundamentalist religious believer says that the Day of Judgment will come, is this the same kind of thing? It seems like a bad idea to lump these two things (ethics and religion) together, and obvious that a believer's use of religious language is not the same as a non-believer's. But it might also be helpful to see some similarities. Ethics and religion are surely pretty well entangled for the believer. And seeing both as lying somewhere between ordinary truth and myth (to each of which, after all, they are connected, I think) might help us avoid an overly literalistic understanding (which might make ethics seem to be mere nonsense) or an overly fictionalist one.
Reading back over that I think I know what I mean, but suspect it might be all a little too obscure for anyone else. Maybe some examples will help. Let's say I am outraged by someone's behavior and find that the best way I can find to express my attitude is to scream "You will burn in hell for this!" at them. If I am, say, a Christian, then I might well mean this literally (what this means will vary among Christians, but never mind that for now). If I am not a Christian then I certainly won't mean it literally. I might be able to find (or be given) some other form of words that say what I mean just as well, or even better, in which case my reference to hell will have been merely metaphorical. But if no other words will capture my meaning (and I don't mean them literally), then I have used them in a secondary sense which, though not metaphorical, is related to literary uses of language and hence to fiction, myth, "higher" truth, etc. (For more on the concept of secondary sense see MKR's helpful comments here).
If I am in that position, the position of having no language but that of a religion to which I do not subscribe, then clearly my beliefs and/or attitudes are connected to that religion (whether I like it or not). This hardly means that Christianity is reducible to questions of attitude and language, but it does show that it is related to such questions, perhaps importantly so. It is also connected, of course, to various historical ("ordinary") facts, but this does not show that it is reducible to that kind of thing either. But I'm trying to talk about ethics rather than religion.
What about "slavery is wrong," "genocide is wrong," or "murder is wrong"? Each of these sentences sounds trivially true and hopelessly inadequate to the truth and so inadequate as to be (if only very slightly) funny. They are (at least somewhat) like "eating people is wrong," which (as the title of this novel) is an actual joke. Of course they wouldn't be equally funny in all contexts. Bewildered and despairing at an actual act of genocide, saying "Genocide is ... wrong!" would not be funny at all. But neither would it hit the nail on the head. It is an inarticulate sentence. (Of course inarticulacy is entirely understandable in some situations, and might communicate your emotion or attitude very well. But showing that you are upset by being inarticulate is not saying that you are upset.) Thankfully (is that the word?) there is a lot more that we can say about the evils of genocide, slavery, and murder than this. Some of this might involve secondary uses of language, but much of it need not. So I don't think we need to regard moral discourse as a kind of fiction or pretense (or mistake).
Another thing to say about such evils is that for most of us they are unthinkable in a pretty literal way. They are not options we are capable of taking seriously (which is surely related to the comic aspect of "Eating people is wrong," etc.). And this is not a truth about the words we use but about thoughts that we don't have. Part -- not all -- of the horror of such things is the discovery that we live among beings for whom such things are thinkable. (Hume sort of suggests that this horror is moral disapproval, the feeling we get when we wonder what kind of person could do such a thing. But moral disapproval is a judgment, not a feeling.) This negative and psychological aspect of moral discourse is something that Joyce does not really address. But what we don't say, and why, seems important to me.