Liberty is primarily a libertarian concept. In fact, you could say it is THE libertarian concept. But it is a concept often borrowed and used by others.
Just as you are not a chess player if you make up your own rules for playing the game, regardless of whether or not you use the proper board and pieces, you are not a libertarian unless you advocate maximum liberty and minimum government. For the vast majority of libertarians that means living by the Zero Aggression Principle as well as you can, and accepting that you have violated your principles if you fail on occasion.
Yet, there will always be quibbles about the meaning of “maximum liberty and minimum government.” It is generally a matter of degree, but there are limits to how far you can stretch the concept without it tearing.
Personally I see maximum liberty to be the freedom to do anything that does not cause physical harm to anyone else or their private property, and does not use deception, theft, or coercion to separate others from their property. I see minimum government to be self government, also called self control. I see the existence of a statutory institution (a “state”) as far above and beyond the preferable minimum government.
If someone or some group claims to be libertarian, but is advocating less than maximum liberty, or is excluding some people from those it considers worthy of maximum liberty, then that individual or group is not living up to its libertarian principles.
If an individual or group self-identifies as libertarian, but is advocating some amount of government in excess of the minimum, whatever that may be, then they are also falling short of their libertarian principles.
If some “libertarian” is excusing theft, coercion, or the rationing of inalienable human rights, for any reason, by rogue individuals or by duly-elected governments, then they are betraying their true nature and giving the lie to their professed principles.
Not everyone who claims the label libertarian is. Nor do all those who give lip-service to “liberty” or “freedom” understand the concepts, or practice what they preach. In fact, some of those who cry “Freedom!” the loudest are its worst enemies.