Well it's not saying "nothing should be enforced ever because all enforcement is backed by violence", it is just acknowledging the reality of it and then determining if a particular rule justifies that, and then farther down, if a rule enforced by violence is unjust, determining whether the enforcing party is an enemy or not.

Generally libertarians only allow for rules against behaviors that violate the rights of others because of this, and I think that's about right.