The United States is protecting the sovereign immunity of another country in order to protect that of its own. If the Vatican can be sued on these grounds then an international precedent would be established that would allow the United States to be sued for its actions and those of its personnel over seas.
Even so, this case deals with instances where it is being argued that the Vatican was acting with knowledge.
In cases of accident or someone acting without knowledge or acting in ignoreance I don’t see how this would set precedent.