Not exactly. There's no real pre-defined Litmus test for what it means to be a True Scotsman... the whole point is that the definition is made up post-hoc. But there is a meaningful, albeit not absolute, litmus test for what it means to be a Libertarian, and that is adherence to the Non-Aggression Principle. Unfortunately the whole discussion is a bit muddled by the typical "big L" Libertarian vs "little l" libertarian dichotomy, and there is nuance to what it means to adhere to the NAP. But saying there is some nuance to the distinction is not the same thing as a No True Scotsman argument.