How do we know the tests are sufficient, that UPB does not give the green light to bad ideas? The way the process works, if a proposition contains a logical contradiction or performative contradiction, I can see how it is reasonable to say it is false. But just because a proposition does not contain one sort of flaw, does not guarantee it does not have another. How can we be sure that Stef has covered all the possible flaws with his tests? A perfectly valid logical syllogism will be true only if it's premises are true.
It is easier for me to think of a moral proposition as false than to know what it would mean for a moral proposition to be true. Certainly Stef is not saying that moral propositions that pass the UPB test can never be violated by anyone, like laws of physics. My best interpretation is that Stef would define truth for moral propositions like this: A moral proposition is true if it does not imply that persons must (may?) violate the norms and assumptions on which argument relies. Argument provides the means by which we communicate, and Stef's approach uses that as a foundation on which to build. A stronger way of putting it would be that true moral statements can be derived from or at least do not contradict the norms and assumptions underlying debate.
If I somehow learn something I believe to be true, but the implications of this truth include the idea that I cannot or should not communicate it to someone, or it is meaningless or pointless for me to do so, can it really be true? (Hoppe and Habermas have taken similar approaches.) This contradiction invites us to conclude that the proposition must actually be false, no matter how much the proposition may tempt us to believe it. If we can only communicate an idea by embracing hypocrisy, there's something wrong with that idea.
If a debater uses coercion to win a debate, at the instant she uses that tactic, the debate ceases to be a debate. Coercion cannot take part in debate. Coercion transforms debate into indoctrination, interrogation, threats, or brainwashing.
This approach may owe more to Hans-Herman Hoppe http://en.wikipedia.org/wiki/Argumentation_ethics than to Stefan Molyneux, I'm not sure.