Stef sees UPB as a general category that includes ethics, aesthetics, math, logic, science, and fair argument.
UPB ethics should be closely analogous to mathematics. Math can be correct, or incorrect. It has different branches that deal with different subjects, and sometimes controversy breaks out among mathematicians over some subtle issue. But it's not possible for two people to have different mathematics. If they disagree, either both of them are wrong, or one of them is wrong, or the topic they disagree about is not really mathematics. Eventually the controversy must be resolved in some way, perhaps in the synthesis of the two ideas.
Ordinary people usually speak as if that is true also of morality, though there is a lot less agreement about the details, much less tendency to converge to a consensus. Even philosophers and sophisticates who espouse moral relativism or subjectivism often continue to use moral language which implies objective, absolute and unique solutions.
Stef claims to defeat the "is-ought" problem with a flanking maneuver. There is an implicit goal in science, the desire to know the truth. The other categories of UPB must have analogous (perhaps identical?) goals. So this provides a "sort-of" solution. Nothing obligates anyone to obey the rules defined by UPB. But if someone guilt trips you, you can use UPB to debunk them. By aiming an "ought" at you, they declare themselves part of a discussion that takes objective truth and universal preferences for granted, and if you catch them in hypocrisy, they fail. By wanting to know what you ought to do, you declare yourself to be part of the group bound by the rules.
What does it mean to say that a moral proposition is false? According to Stef, a moral proposition is false if it can be shown to contradict the norms of fair argument that I must accept before I can participate in the discussion, or if it contains a logical contradiction.
Is this fair? Is it possible for the following statement to be true? "I am a liar and 2 + 2 = 4." Stef makes an analogous argument for UPB ethics. He claims that for someone to deny the norms of argument while making an argument creates a performative contradiction. Yet, we can tack on any number of valid arguments at the end of their denial, and the arguments remain valid. It is the entire argument as a whole that is false, not necessarily each of the component claims. So perhaps all we can say of such arguments is that their truth value can't be determined in the usual way.
On the other hand, if I was unable to say "2 + 2 = 4" without denying that truth is objective, that would raise a red flag. Maybe my analogy lacks something, specifically, the two parts of my claim seem completely independent of each other, but the UPB skeptic's critique is inseparable from the context of argumentation.