I remember a libertarian telling me once that they believed it was unethical to start a war in order to free slaves, because violence is only ok in self defence and the slaves could rise up on their own.
That's not in line with the NAP. Slaves are by definition the subjects of aggression since they have been stripped of their natural self-ownership right. So it is definitely not unethical to help them.
To my mind, "not in line with" implies that it'd motivate violation of the principle. It seems like it's rather a stricter (... which doesn't necessarily mean better or worse) standard than the NAP (or at least the most common interpretations of it).