Within the principle, it defines its terms. Aggression is not defined as the somewhat vague, textbook definition that is presented here.
That's true - Among people familiar with the use of the term in this context, this is no problem. But the ancap use of the term Aggression is misaligned with mainstream understanding of what the term denotes. I think this is at best a barrier to understanding for non-initiates, and at worst, a misleading term to use. The Non-transgression Principle looks like a superior alternative without these problems.