It isn't semantic, they're appropriating a term that doesn't apply to them.

Are you for real? The term libertarianism was used to describe specific factions of socialists (like the mutualists) around 100+ years before it was co-opted by right-libertarians in the US, and the anarchists never stopped using it for themselves either in europe or in the US. To turn this around and attempt to claim the original libertarians are attempting to usurp the word from the usurpers demonstrates a serious lack of understanding of its history. They are not slapping "libertarian" on their title. They practically invented the term in the context of political philosophy.

but are actively anti-capitalist, which means they don't really accept the principles of liberty.

As defined by whom? Obviously anti-capitalists don't accept the same understanding of liberty as that promoted by the Mises crew, but that doesn't follow from a nominal rejection of "the principles of liberty". They disagree on what those are and to claim that's just lip service reveals a very uncharitable reading of the literature or ignorance thereof. What freedom is, whether it's categorical or maximisable and what it entails as a category is a very complex debate that is currently developing on a meta-theoretical level. It's not solved and it's definitely not the monopoly of one small fraction of the people that are arguing about it. In fact, with the formal development of political republicanism and the resurgence of neo-kantianism it's not even a monopoly of those more or less critical of the state.

What they really do is make unfalsifiable claims

So what? Are you under the impression Nozick is making falsifiable claims when he's arguing for his principles of justice? Whether falsification is possible in abstracto notwithstanding (Quine's underdetermination thesis would be relevant here), political philosophy is not a science, it's not aiming to make predictions about future events, but to reason about justice.