It matters because of the fact/value dichotomy. One cannot get to an ought-statement from an is-statement. Hoppe places special emphasis on this divide in his argument.
My reading of ethics, so far, is limited to what I’ve read from Hoppe (in other words, works which I’ve read purely for insights into ethics; every political tract deals with poltics, which is a branch of ethics [that branch concerned with the legitimate use of force]) so my knowledge of ethics is limited (although, thanks to this thread, I now have a pretty good ethics reading list). Hoppe takes care at the outset of his argument that he is dealing specifically with ethics as the logic of justifying arguments; he separates his argument from the branches of ethics that deal with how one should live their life on the basis of what one thinks to be the case (contained in this branch includes arguments based on, but not limited to, religious teachings and emotive arguments). A rationalist, Hoppe attempts to provide an objective, rational basis for ethics.
Awesome.
Yes you are contradicting yourself: the content of this proposition explicitly holds aggression to be legitimate while you deliver the proposition assuming, by what is implied by the act of argument, the validity of private property rights and the NAP.
An example of this proposition in action would be the following: I meet you on the street. I have never before met you in my life. You punch me in the face and steal my wallet (… jerk). What you did is argumentatively unjustifiable (for reasons mentioned above).
NB: Hoppe does not deny that there have been anti-libertarian actions in the past (this is one of the objections to his argument); all his argument states is that such actions are argumentatively unjustifiable (as is aggressing against anyone with or without argument in doing so).