the position that holds that a discourse typically is used in an assertoric manner [i.e. to make assertions], but those assertions by and large fail to state truths.This means that if you think most of the assertions in a particular discourse (or language game, if you prefer) are false then you are an error theorist about, or with regard to, that discourse. But you would equally be an error theorist if you thought the assertions were neither true nor false, or if, like the early Wittgenstein (Joyce is explicit about this) you think they are nonsense (as long as you think the discourse is still assertoric).
Moral discourse (think of sentences such as "Slavery is wrong" and "The Nazis were evil") seems to be assertoric. Sentences in works of fiction also look like assertions ("Someone must have slandered Josef K., for one morning, without having done anything truly wrong, he was arrested"--although some of the examples found here are not obviously untrue in this way), but if you asked someone reading these sentences whether they were really true you might well get the response "Of course not." That is not the response you would expect if you asked whether slavery is really wrong or whether the Nazis were really evil. Quite the reverse. So moral discourse is not fiction, and Joyce's fictionalism should not be taken to claim otherwise.
The next chapters argue that moral discourse as we know it consists largely of assertions that are not true though.