I think I saw elsewhere that the owner has been the same for 35 years or something like that. So there is no relevant transfer of ownership.

Your opening statement here is pretty ridiculous I need to say. "Written agreements doesn't mean shit!" Then you just make up a scenario where the contract wouldn't hold up.

They do not define what their written or verbal agreements are, it is as simple as that. That statement would mean its possible they asked 35 years ago got a shrug and some guy saying "sure" OR it could maximally be a full contract with permanent transfer of the right of painting the wall. If this organization uses a vague statement, then its fair to take the vagueness is the worst possible way for them.

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.