1. If belief in a proposition is inconsistent with being able to defend it argumentatively, the proposition is false.
2. In order to argue about the truth of proposition we must have absolute self ownership and ownership of scarce means, defined in objective, physical terms and obtained via homesteading.
3. The denial of a libertarian ethic is false.
So far as I can see, both 1 and 2 are false. With regard to 1, consider the proposition "One should never argue about what people should do." Belief in it is inconsistent with defending it argumentatively, but that tells us nothing at all about whether it is true or false. One could even imagine someone who did not believe in the proposition constructing a valid argument proving that it was true, although he would presumably stop speaking as soon as he had completely convinced himself.
As to 2, note that if it is literally true nobody, including Hoppe, has ever argued about the truth of propositions, since there are no completely libertarian societies in which they could do so. That is obviously not true--and neither is the proposition from which it follows. One can think of an enormous number of non-libertarian ethics and non-libertarian societies consistent with people being able to argue in their defense.
Consider an ethic according to which people have absolute ownership over half their waking hours, and are obliged to spend the rest working for others--eight hours a day is enough time for quite an extensive philosophical argument. Or consider an ethic according to which we are obliged to spend all our time working for others, but defending that ethic classifies as working for others.
As a final example, consider an ethic according to which there are no rights at all; everyone is morally free to coerce everyone else whenever he can get away with it, but many people succeed in defending themselves well enough so that they control much of their own time. According to their ethic they have no right to self ownership, nor to anything else, but they have physical control over themselves and are therefore able to make arguments. One might plausibly claim that this comes close to describing the world we now live in.
The extension of 2 to cover not only self-ownership but libertarian property rights as well, and even a particular libertarian theory of what property rights are like and how they are acquired, is if anything still less defensible--almost pure assertion, unleavened by argument. One can think of lots of other systems of property rights that would work at least well enough to keep some people alive to argue philosophy. Hoppe has somehow skipped from "your ethic must allow you to live" to "your ethic must do the best possible job of letting people live" to "you must accept Hoppe's preferred form of libertarianism" (via "Hoppe's preferred form of libertarianism does the best possible job of letting people live").
Counter-examples include all societies that have existed for as long as one generation, since in all such societies people did in fact live long enough to grow up and argue philosophy, and none of them were pure libertarian societies.
"The Trouble with Hoppe," Liberty 2.2 (Nov. 1988): 44. Reprinted by permission.