That was just one example of how there can be different kinds of agreements, as long as everyone's on the same page.
Right, everyone agrees. Voluntaryism. Property norms must exist in for the cases when everyone does not agree. We cannot expect everyone to magically agree on how to share scarce resources needed for survival. This can be objectively demonstrated through logic and observation of real life societies. Even if everyone agrees in a commune to share things in a certain way, others from outside may have a different "agreement" and decide that they cannot share in that way, and that the resources they are sharing are now theirs. The same way a group of Communists might say land ownership is not valid and destroy a peaceful propertarian community because hey they can't own that land!