His point is that "written and vegan agreements" is not the same thing as "has a contract" and that the legal niceties need to be respected or else it would be impossible to get anything done.

Asking "is it cool if I paint your wall?" more than three decades ago doesn't create a contract or protect the work.

I highly doubt that street artists made a legally binding agreement with the owners that protects the art for the simple reason that this would be a ridiculous encumbrance on the property that would be unacceptable to most sane people.