I don't necessarily agree with the person you responded to, and I could be wrong here but I don't really think Banksy is actually invoking their copyrights, just using it as an idea to criticize private property in general. Similar to how your own "god given copyright" is in itself a criticism. It's more like, "look our property laws that are meant to protect the art-maker mean nothing to big companies. Why should the property laws that are meant protect big companies mean anything to us?"

I get how you could see it as hypocritical, but I think fundamentally Banksy probably isn't advocating for stronger copyright laws here...