Open borders, in a general sense, are difficult to reconcile with anarchocapitalism, since one of its core principles is private property. To which author are you referring?

It’s also not accurate to describe anarchocapitalism as morally empty. At its core is the principle that any initiation of aggression against a person or their property is illegitimate (not just for the state)

Also one could argue that anarchocapitalism creates the preconditions for morality in the first place (negative rights/freedom). Moral action presupposes freedom of choice: if individuals are not free to choose their actions, then their behavior cannot be described as moral or immoral.