Most importantly, it comes from thinking about why the current laws and conventions are justified. They are not simply there in reality. Why do we have them?
It is not about justifying piracy, it is saying we should not even have the laws in the first place that lead to the possibility of piracy.
You might reply that this is ridiculous: simply changing the name of an act does not remove the harm it does. But this is the core of the matter: with copying, that reply is basically false! (Piracy is in legal terms a malum prohibitum not a malum in se.)
Look at the underlying phenomena, prior to any legal construction. Copying is an abstract relation: there is no physical connection between an original and a copy. It no more harms someone to be copied than it harms the number 4 when you write it down. Since copying renders no harm, and in fact the opposite, it is very beneficial, and furthermore it is an abundance, why should we restrict it? (No moral argument for copyright/IP/etc. seems ever to have answered that.)
In practice it may possibly be that selling creative effort is difficult to do optimally without some structure like copyright etc. That is the contention of the standard economic view. But that gives only a very particular and limited justification. Such restrictions on copying are justified only insofar as they actually turn out to help us overall.
Which returns us the gist of the article: if those restrictions do not help much, or rather have comparable alternatives, the justification for their existence is gone.