I don't see how pointing out that this is inconsistent with the principles of libertarianism means that this is a "no True Scotsman" fallacy.

Libertarian principles are pretty straightforward, even though there is some variation in just how little coercion there is and where it's allowed to come from. Libertarianism with slaveholding makes about as much sense as democracy with a hereditary absolute non-ceremonial monarch. It contradicts the premise of that form of political structure. That is: the slaveholding part is in drastic contradiction to the libertarian part (even if other aspects are libertarian), just as the absolute monarch would be in drastic contradiction to the democracy part (even if other aspects of the society are run democratically).

Words mean things. I'm not using the example to define them (that's where the fallacy comes from). I'm just using the standard definition(s).