I would stand by the statement that most secular morality is subjective when it comes to people like Thunderf00t and the other Utilitarians like him. If you want objective morality, you need to arrive at moral conclusions via deontological methods, much like how Immanuel Kant did. However, like you said, you could never do this using logical positivism in order to create a moral theory. Instead, I use falsification, which is the basis for my moral argument from Liberty. Because all claims of legitimacy have thus far have been falsified, Liberty is the default winner since it means nobody has a legitimate claim to authority. From this insight, we can logically deduce the non aggression principle and the fundamental rights (a.k.a. the Negative Liberties); this is the only rational conclusion you can reach if nobody has legitimate authority over anybody. Everything else is neither moral nor immoral, but personal preference.
I haven't ready UBP, but I have seen one of Stefbot's lengthy videos on the subject and I must say that it disappointed me simply because it was based on logical positivism. It very well could be my own bias, but I feel that my moral theory, based on falsification, is far simpler and not invalidated by the problem of verification. If he modified his position, he would probably come up with a solution like mine, which itself is based on the political theories of John Locke. I doubt that he would, though, unless he gave up his logical positivism. I think he craves certainty too much in order to do that, based on his videos.
I agree that just because people hold an opinion does not make it true by necessity; this should be intuitively obvious. However, when a significant number of people hold an opinion or belief, it is proper to fully understand the opinion or belief in question before attempting to falsify and dismiss it. Opinions, just like theories, are not always completely wrong when falsified. Some aspects may be true even if it is mostly false, so you always want to be careful not to throw out the baby with the bath water, as they say. It is also helpful to understand why people have a particular belief in the first place, too, as this will give powerful insights into how bad information can be falsified.
You seem to think that the statement: "Absolute Truth (with a capital T), if it even exists, is unknowable." is a statement of knowledge, and it is. In fact, the only thing you can know with perfect certainty is that nothing else can be known with perfect certainly. I would caution you, though, to understand that knowledge deals with empirical facts, not truth. Remember that empirical facts are learned through sense data and are thus a perception of what actual reality is. All of your knowledge is, in fact, a perception of reality, not reality itself. So yes, saying that absolute truth cannot be known is a statement of knowledge, but it is not a statement of truth. Truth pertains to the unknowable reality.
Just in case I haven't mentioned this already, you may be interested in checking out College Binary's channel on YouTube. His channel has a feature called 3 minute philosophy which covers many of the great philosophers throughout history. He also has an excellent Libertarian bent. He's got lots of good stuff that is pretty spot on. I found his channel after taking a real philosophy class, so I should know.http://www.youtube.com/user/CollegeBinary