Throughout his career, Rothbard harshly criticized the modern neoclassical approach to welfare economics, which considers reallocations "efficient" so long as they are "potentially Pareto superior." While the justice of efficiency is far from evident, this criterion of efficiency has many advantages over Rothbard's approach.
So it looks pretty unjust, but it's better than Rothbard's. And why?
In particular, it actually allows one to make efficiency judgments about the real world - to judge, for example, that Communism was inefficient, or rent control is inefficient, or piracy was inefficient.
Not sure what he means by inefficient here. If he means some artificial use of the word, like Kaldor-Hicks efficiency, or Pareto efficiency, well yes. Rothbard's approach does not measure these things. But if he means by inefficient "achieves the opposite of its intended goal", then AE has extensive literature to show the guaranteed failure of Communism and of rent control. As for piracy, do we really need complicated mathematical theories to prove it is not a viable economic system?