I have a libertarian in my family. He says that on the matter of defining who is and who is not a libertarian, it’s best to read The Anarchists’ Convention by John Sayles.
So, I guess, in defense of libertarians (did I just say that?) I’ll say that at least some of them are very aware of the irony of a libertarian saying someone else isn’t a real libertarian (while still doing it).
ETA: And of course they still do it, it can’t be helped. I can’t help saying some people aren’t real libertarians. There was a candidate for leader of a state Libertarian party who wanted to bring back prohi-fucking-bition.