In a sense, he’s right: international law has no direct enforcer.
However, international law is more the kind of rules existing in a pre-state society, born from global consensus between individuals deciding on which rules are best for the common good: for exemple, a community might enforce grazing limitations in its commons.
Hell, even gangsters and especially organised crime has rules, because they know that negociation, consensus and compromise is better for their business (not to mention producing less heat) than “going to the mattresses.”
Hugo Grotius already wrote treatises on international law and even Ancient Rome recognized jus gentium. These last decades, with how mankind-endangering can be some weapons and how destructive is industrial war, its importance grew even more.