The author counters this claim by saying in a libertarian society, "Imagine, for example, a perfectly equal society, in which everyone had access to the same educational opportunities. In such a society, no one could complain that the choice to enlist in the military was less than free, because unfairly pressured by economic necessity." (Chapter 4 , Objection 1: Fairness and freedom)
This claimed became debunked since in a perfect society, there couldn't be social and economical divides as large as today.