Apologies for the delay in responding to your query (and many thanks for the compliment re the course).
The first thing to say is that H-H Hoppe’s contribution to Libertarian thought is in no way confined to his account of argumentation ethics. From his Democracy: The God that Failed to his The Great Fiction, his libertarian writings (whether one agrees with them in whole or in part or at all) are a hugely significant contribution to the ongoing investigation of the implication of liberty. I never read anything he has written without becoming aware of implications and nuances that I had not perceived before. To take one example, his appropriation of the Marxist doctrine of class warfare (which, of course, antedates Marx) and his application of it to Libertarian ends is really insightful.
Argumentation ethics (AE): here, I’m conflicted. I have examined the arguments pro and con (at one stage, I assembled notes and comments running to about 30 pages or so on the topic) but I wasn’t able to come to a definitive conclusion. AE is an instance of a kind of argumentation sometimes called transcendental. Some thinkers are allergic to this kind of argument in general but I’m not. In fact, in my Libertarian Anarchy, I use just such an argument to argue against strict determinism.
M: No theory can be seriously maintained such that, if it were to be true, its very maintenance would become impossible, meaningless, contradictory or self-refuting.
Apart from the formal constraints on theories of the necessity for consistency and coherence, and the material constraints of explanatory adequacy and coverage, there is also a self-referential constraint on theories, namely, that theories must not render impossible the conditions of their own statement or the conditions of their being maintained. If they do so, they are theoretically self-stultifying. Unless human beings are fundamentally free in their choices and decisions, it is not possible for statements to be meaningfully asserted: that includes the statement of a radical determinism or a radical irrationalism. The statement of a radical determinism is undermined by its own content’s rendering pointless the act of its assertion or by its assertion’s rendering meaningless the content of that assertion; the same holds true for the statement of a radical irrationalism. Iris Murdoch writes: “As a philosophical theory, as contrasted with a theological view or an assumption of popular science or an emotional intuition about fate, determinism fails because it is unstateable. However far we impinge (for instance for legal or moral purposes) upon the area of free will we cannot philosophically exhibit a situation in which, instead of shifting, it vanishes. The phenomena of rationality and morality are involved in the very attempt to banish them.”
Strict determinism falls foul of the maxim since, of necessity, the very attempt to argue for determinism is itself a free act by the arguer which commends itself to the rational judgement of its intended audience. If it is not a free act, we need not regard it; it is only the sighing of the breeze in the vocal chords of the determinist. Irrationalism, on the other hand, while not quite as neatly self-destructive as determinism, is nonetheless obviously rationally unsustainable.
While accepting of transcendental arguments in general, however, I’m not sure if AE is immune from criticism. But then, very few serious philosophical arguments are so immune but that doesn’t mean that they aren’t taken seriously by philosophers.
I wish I could be more definitive in my answer to you query but I hope that I’ve made it clear that I think that Hoppe’s AE cannot be summarily dismissed.