Yes anyone can call themselves libertarian and many leftists from Chomsky to Bill Maher are libertarians, especially since it is mostly leftism anyway, that is why conservatives are always being told to move left in elections so that the GOP can win their vote, because all that social conservatism and strong defense will make them go back to the democrats.
That is why the libertarian party is the closest we can come to defining what libertarianism is, outside of their official party, individuals make all kinds of self definition and you can’t argue with mercury, all you can do is listen to a single sided discussion as each individual lectures you from their own personal vision of what they are, you can’t challenge because they merely tell you that “no, that doesn’t fit my definition, I am on the opposite side of those issues”.
The Libertarian party is libertarianism in practice and on the ballot.
Patently untrue.
It is the Establishment parties which are "mostly leftism..."