The argument relayed in the question is trivially true
and also, irrelevant as regards the debate over piracy.
At the micro-economic level, piracy has to do with violation of property rights, and so with distribution of output, not with its level (to which the argument in the question alludes). It has to do with the principle
"This is mine, and if you don't pay for it, you have no right to consume it."
While in... principle, this is a principle that is accepted by most humans on earth, it has been observed that when the entity saying this is perceived as rich/wealthy etc, people start to question (even if indirectly) whether the principle should be upheld to the maximum: art piracy is "felt" by many as a rightful redistribution (and this is why, strategically, the Entertainment Industry tries to shift the focus of the debate to the negative effects as regards loss of jobs etc, against which an approach like the one relayed in the question is a valid counter-argument).
At the macro-economic level, a widespread disrespect of, and the failure to protect the, property rights, leads to increased uncertainty and reduced output, because economic agents will start spending considerable and disproportionate resources to protect their property rights (on wealth and on production), and hence withdraw these resources from the production of output itself. The fact that in National Accounting these protection services will turn up like production themselves, does not change the fact that the other forms of output will be lower. Such protection services are already widespread: most of the legal system and the military expenses are about protecting property rights at one level or the other, as are private security services.
In an economy where property rights are more or less adequately protected, piracy can be said to also affect the composition of output. Who, and through which means, gets to influence how many films and how many skateboards will be produced? And when these influences come partly through the violation of such a fundamental social principle as rights to property, do they have a detrimental effect?