This is sort of like the same phenomena of the politically correct (not in the PC sense but in the sense of like, what is and isn't like, correct in the realm of political discourse. Like definitions of semantics and shit) definition of liberalism that leftists have to kind of churn through and give, every time someone says liberals and leftists are the same thing, and then it's explained in some sort of hackneyed way usually that "on the global scale of leftism actually you're wrong sweetie", when realistically the better way to describe it is that liberalism isn't necessarily left or right wing because it's kind of a mercenary ideology that leaves up a free market which may either be left or right wing, depending on circumstance.
And then everyone gets confused by that distinction between liberalism and leftism, and just go back to using the words how they were using them to begin with, and calling people libtards, despite themselves wanting a free market more than their opposition (usually). So what I mean to say is that your definition is technically correct by all given definitions, and is the only one that makes sense, right, but, despite that, when most people refer to libertarians, they're referring to this exact type of twat who drives a yuge truck, is generally obsessed with firearms, may or may not be a pedophile who doesn't like the age of consent, may or may not be an austerity hawk, and believes in the NAP as some sort of holy preventative doctrine that you can build a society on. Hackneyed, conservative-flavored anarchism, basically. That strain of conservatism where they actually believed Reagan when he said the enemy was the government. That's what people mean when they say someone's a libertarian, and it's usually also what people mean when they self-define as a libertarian.
It's not a technically correct or logically coherent definition, but it's the one that's worked it's way into common cultural parlance.