I'm not sure what you mean by saying the law is notoriously slow to adapt, since Facebook is basically quoting the law to the user.
Section 4(12) of the Acts carves out an exception to
subject access requests where the disclosures in
response would adversely affect trade secrets or
intellectual property. We have not provided any
information to you which is a trade secret or
intellectual property of Facebook Ireland Limited
or its licensors.
For all I know, Facebook may even be obligated to include such a disclaimer about missing information when processing an information request.