They say "He's not a libertarian!"
Saying “he’s not a libertarian” is just a way by which Libertarians dissociate themselves from a candidate they think will damage the reputation of their movement. It has little or nothing to do with the candidate’s stances.
An example would be how Paul Stanton, who ran in the Libertarian primary against Augustus Sol Invictus, said, “Mr. Invictus’ brand of neo-fascism does not represent Libertarian values.” If you actually look at Invictus’s platform, it’s pretty much the same as any other Libertarian candidate’s, with its talk of ending the drug war, getting rid of the Federal Reserve, etc. There’s nothing in there about invading Poland or gassing the Jews.
But Libertarians want to distance themselves from him because the controversies over his Paganism and whatnot might embarrass the Party. So they say, “He’s not a libertarian.” The fact that they refuse to nominate him helps make their claim more believable to the public, compared to when, say, Christine Smith, after losing to Bob Barr in the contest for the 2008 Libertarian nomination, said that he was not a libertarian.
Basically, what counts as “libertarian” is determined by a vote of the Party. If they wanted, they could nominate a communist and declare that he’s the “Libertarian” candidate, and say that the non-communist candidates “aren’t libertarian” because they support policies, such as abolition of progressive taxation, that favor the rich at the expense of the poor. They can interpret the self-ownership and nonaggression principles in such a way as to mean that anything they want is or isn’t libertarian.
But let's remember, there are at least 24 kinds of libertarians (just like there are at least 24 kinds of authoritarians). So just because I'm not the same kind as you, doesn't mean I'm not a libertarian.